Added one more example with guards, fixed typos and identation.

elixir.html.markdown

@@ -246,8 +246,8 @@ end
 Math.sum(1, 2)  #=> 3
 Match.square(3) #=> 9
 
-# To compile our little Math module save it as `math.ex` and use `elixirc`
-elixirc math.ex
+# To compile our simple Math module save it as `math.ex` and use `elixirc`
+# in your terminal: elixirc math.ex
 
 # Inside a module we can define functions with `def` and private functions with `defp`.
 # A function defined with `def` is available to be invoked from other modules,
@@ -278,6 +278,8 @@ end
 
 Geometry.area({:rectangle, 2, 3}) #=> 6
 Geometry.area({:circle, 3})       #=> 28.25999999999999801048
+Geometry.area({:circle, "not_a_number"})
+#=> ** (FunctionClauseError) no function clause matching in Geometry.area/1
 
 # Due to immutability, recursion is a big part of elixir
 defmodule Recursion do