fixed incorrect syntax highlighting language

The example codeblock for JavaScript code was using `javascript` as the
syntax highlighting, which is obviously wrong, it should be `js`

CONTRIBUTING.md

@@ -25,7 +25,7 @@ Single use functions are also generally discouraged, if a function is used once
 
 As shown below below:
 
-```javascript
+```js
 function foo(bar) {
 	/*
 		This is a very long comment, however, it does not exceed 72