Add a doc heading for the ActiveModelSerializer class to so its page sho...

...ws up in the API docs

This is a fix for https://github.com/emberjs/website/issues/1531

![screen shot 2014-05-21 at 6 58 42 pm](https://cloud.githubusercontent.com/assets/54056/3047823/df1d77ca-e13b-11e3-8c91-a69ec5adc2d0.png)