[DOC beta] Fix syntax highlight

I fixed some syntax highlight on code comment.

### `Ember.on`

Before:
![2013-12-20 2 09 45](https://f.cloud.github.com/assets/290782/1784288/7a2f7dec-68d0-11e3-8cb3-dd6a2557122a.png)

After:
![2013-12-20 2 09 51](https://f.cloud.github.com/assets/290782/1784286/7a2d0544-68d0-11e3-9045-59985afe373d.png)

### `Ember.assert`

Before:
![2013-12-20 1 50 01](https://f.cloud.github.com/assets/290782/1784287/7a2f22fc-68d0-11e3-8d5a-9717fe4d6d05.png)

After:
![2013-12-20 2 08 14](https://f.cloud.github.com/assets/290782/1784289/7a541166-68d0-11e3-936b-9e415a7bfcef.png)

### `Ember.onload`

Before:
![2013-12-20 2 07 29](https://f.cloud.github.com/assets/290782/1784291/7a58553c-68d0-11e3-8df0-c858fb32dcda.png)

After:
![2013-12-20 2 07 16](https://f.cloud.github.com/assets/290782/1784290/7a569756-68d0-11e3-8b98-ef4f2709ccd0.png)