Cleanup Headers

Just two wee changes here:

- _Classes -- Extended_: didn't include the 'Extended' bit on the title that's displayed above the intro
- _Array Destructuruing_: displayed the title twice

Have I mentioned how awesome this is, @BrianGenisio? Great work! This is exactly what I set out looking for this past weekend... an easy sandbox to play with some ES6 concepts. I think we should pretty it up a bit more and try to get it some more exposure... maybe [JavaScript Weekly](http://javascriptweekly.com/) or something.