[apl/en] format correctly (#4938)

apl.html.markdown

@@ -1,6 +1,11 @@
-⍝ These examples can be tested here:
-⍝    http://ngn.github.io/apl/web/index.html
-⍝
+---
+language: APL
+contributors:
+    - ["nooodl", "https://github.com/nooodl"]
+filename: learnapl.apl
+---
+
+```apl
 ⍝ Comments in APL are prefixed by ⍝.
 
 ⍝ A list of numbers. (¯ is negative)
@@ -55,3 +60,4 @@ A ← 10 60 55 23
 ⍝ We can define this as a function using {} and ⍵:
 mean ← {(+/⍵)÷⍴⍵}
 mean A                   ⍝ 37
+```