[DOC beta] fixes formatting

Before:

![ember js - ember mutablearray 2013-12-20 12-48-09](https://f.cloud.github.com/assets/1131196/1792394/e4522390-699e-11e3-9d78-7669aa92db46.jpg)


After:

![ember js - ember mutablearray 2013-12-20 12-47-33](https://f.cloud.github.com/assets/1131196/1792392/cecf9b10-699e-11e3-84d7-903c093b4a15.jpg)