Update .cocoadocs.yml

![screen shot 2014-10-14 at 11 54 58 am](https://cloud.githubusercontent.com/assets/49038/4632044/77dd4f80-53ba-11e4-9549-e23d1ad14da0.png)

The header is hard to read on [CocoaDocs](http://cocoadocs.org/docsets/AFNetworking/2.4.1/), so I've darkened the colour. In the above example the twitter handle, and the license text are the new colour. Everything else is how it is now.