[DOC] Fix markup for AcriveModelAdapter

`active_model_serializer` will be rendered to `active<em>model</em>serializer` against our expectation.
`_` should be escaped.

Before:
![2014-12-21 3 38 42](https://cloud.githubusercontent.com/assets/290782/5515876/36d15250-88c3-11e4-8b35-fce543be512f.png)

After:
![2014-12-21 3 41 26](https://cloud.githubusercontent.com/assets/290782/5515878/45ce3426-88c3-11e4-9a61-ced9da25c95a.png)