[DOC beta] fixes for docs

Before:

![ember js - ember mutablearray 2013-12-20 12-42-17](https://f.cloud.github.com/assets/1131196/1792361/12b94aac-699e-11e3-88ee-7404e5e291b2.jpg)

After:

![ember js - ember mutablearray 2013-12-20 12-41-39](https://f.cloud.github.com/assets/1131196/1792357/fc076d66-699d-11e3-8928-e8d11d39a277.jpg)