Make style fixes to conform to ruby style guide

Made style fixes to conform to Ruby style guide.
Added a reference to a community-driven Ruby coding style guide.

ruby.html.markdown

@@ -9,6 +9,7 @@ contributors:
   - ["Nick LaMuro", "https://github.com/NickLaMuro"]
   - ["Marcos Brizeno", "http://www.about.me/marcosbrizeno"]
   - ["Ariel Krakowski", "http://www.learneroo.com"]
+  - ["Dzianis Dashkevich", "https://github.com/dskecse"]
 
 ---
 
@@ -78,14 +79,17 @@ false.class #=> FalseClass
 'I am a string'.class #=> String
 "I am a string too".class #=> String
 
-placeholder = "use string interpolation"
+placeholder = 'use string interpolation'
 "I can #{placeholder} when using double quoted strings"
 #=> "I can use string interpolation when using double quoted strings"
 
+# Prefer single quoted strings to double quoted ones where possible
+# Double quoted strings perform additional inner calculations
+
 # Combine strings, but not with numbers
-"hello " + "world"  #=> "hello world"
-"hello " + 3 #=> TypeError: can't convert Fixnum into String
-"hello " + 3.to_s #=> "hello 3"
+'hello ' + 'world'  #=> "hello world"
+'hello ' + 3 #=> TypeError: can't convert Fixnum into String
+'hello ' + 3.to_s #=> "hello 3"
 
 # print to the output
 puts "I'm printing!"
@@ -130,7 +134,7 @@ array = [1, 2, 3, 4, 5] #=> [1, 2, 3, 4, 5]
 
 # Arrays can contain different types of items
 
-[1, "hello", false] #=> [1, "hello", false]
+[1, 'hello', false] #=> [1, "hello", false]
 
 # Arrays can be indexed
 # From the front
@@ -180,11 +184,11 @@ new_hash.keys #=> [:defcon, :action]
 # Control structures
 
 if true
-  "if statement"
+  'if statement'
 elsif false
-  "else if, optional"
+  'else if, optional'
 else
-  "else, also optional"
+  'else, also optional'
 end
 
 for counter in 1..5
@@ -241,32 +245,30 @@ grade = 'B'
 
 case grade
 when 'A'
-  puts "Way to go kiddo"
+  puts 'Way to go kiddo'
 when 'B'
-  puts "Better luck next time"
+  puts 'Better luck next time'
 when 'C'
-  puts "You can do better"
+  puts 'You can do better'
 when 'D'
-  puts "Scraping through"
+  puts 'Scraping through'
 when 'F'
-  puts "You failed!"
+  puts 'You failed!'
 else
-  puts "Alternative grading system, eh?"
+  puts 'Alternative grading system, eh?'
 end
-
 #=> "Better luck next time"
 
 # cases can also use ranges
 grade = 82
 case grade
 when 90..100
-      puts "Hooray!"
+  puts 'Hooray!'
 when 80...90
-      puts "OK job" 
+  puts 'OK job'
 else
-      puts "You failed!"
+  puts 'You failed!'
 end
-
 #=> "OK job"
 
 
@@ -298,9 +300,9 @@ sum sum(3,4), 5 #=> 12
 # it can be called with the 'yield' keyword
 
 def surround
-  puts "{"
+  puts '{'
   yield
-  puts "}"
+  puts '}'
 end
 
 surround { puts 'hello world' }
@@ -313,20 +315,20 @@ surround { puts 'hello world' }
 # You can pass a block to a function
 # "&" marks a reference to a passed block
 def guests(&block)
- block.call "some_argument" 
+  block.call 'some_argument'
 end
 
 # You can pass a list of arguments, which will be converted into an array
 # That's what splat operator ("*") is for
 def guests(*array)
- array.each { |guest| puts "#{guest}" }
+  array.each { |guest| puts guest }
 end
 
 # Define a class with the class keyword
 class Human
 
   # A class variable. It is shared by all instances of this class.
-  @@species = "H. sapiens"
+  @@species = 'H. sapiens'
 
   # Basic initializer
   def initialize(name, age = 0)
@@ -356,20 +358,19 @@ class Human
   # A class method uses self to distinguish from instance methods.
   # It can only be called on the class, not an instance.
   def self.say(msg)
-    puts "#{msg}"
+    puts msg
   end
 
   def species
     @@species
   end
-
 end
 
 
 # Instantiate a class
-jim = Human.new("Jim Halpert")
+jim = Human.new('Jim Halpert')
 
-dwight = Human.new("Dwight K. Schrute")
+dwight = Human.new('Dwight K. Schrute')
 
 # Let's call a couple of methods
 jim.species #=> "H. sapiens"
@@ -380,7 +381,7 @@ dwight.species #=> "H. sapiens"
 dwight.name #=> "Dwight K. Schrute"
 
 # Call the class method
-Human.say("Hi") #=> "Hi"
+Human.say('Hi') #=> "Hi"
 
 # Variable's scopes are defined by the way we name them.
 # Variables that start with $ have global scope
@@ -399,7 +400,7 @@ defined? @@var #=> "class variable"
 Var = "I'm a constant"
 defined? Var #=> "constant"
 
-# Class also is object in ruby. So class can have instance variables.
+# Class is also an object in ruby. So class can have instance variables.
 # Class variable is shared among the class and all of its descendants.
 
 # base class
@@ -451,8 +452,8 @@ module ModuleExample
   end
 end
 
-# Including modules binds the methods to the object instance
-# Extending modules binds the methods to the class instance
+# Including modules binds their methods to the class instances
+# Extending modules binds their methods to the class itself
 
 class Person
   include ModuleExample
@@ -467,7 +468,7 @@ Person.new.foo # => 'foo'
 Book.foo       # => 'foo'
 Book.new.foo   # => NoMethodError: undefined method `foo'
 
-# Callbacks when including and extending a module are executed
+# Callbacks are executed when including and extending a module
 
 module ConcernExample
   def self.included(base)
@@ -504,5 +505,4 @@ Something.new.qux # => 'qux'
 - [Official Documentation](http://www.ruby-doc.org/core-2.1.1/)
 - [Ruby from other languages](https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/)
 - [Programming Ruby](http://www.amazon.com/Programming-Ruby-1-9-2-0-Programmers/dp/1937785491/) - An older [free addition](http://ruby-doc.com/docs/ProgrammingRuby/) is available online.
-
-
+- [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide) - A community-driven Ruby coding style guide.