[DOC beta] Fixes highlighting

Before:

![ember js - ember mutablearray 2013-12-20 12-50-40](https://f.cloud.github.com/assets/1131196/1792418/3de86a7c-699f-11e3-87be-62f3b885cbb4.jpg)

After:

![ember js - ember mutablearray 2013-12-20 12-51-39](https://f.cloud.github.com/assets/1131196/1792431/614b76d0-699f-11e3-9781-7e6e208d19a8.jpg)