Update user provider config in the Facebook connect document.

The original docs doesn't work with master branch in Symfony 2.3 anymore for some reason, and here is the update.

* Now use `hwi_oauth.user.provider` provider instead of `hwi_oauth.user.provider.entity`.
* Add `hwi_oauth.user.provider` to security.yml's `providers` node.  
Without this line it will throw "There is no user provider for user "HWI\Bundle\OAuthBundle\Security\Core\User\OAuthUser"" exception. Other docs didn't mention this part I think.