Update documentation -> user provider settings

Current versions of FOSUserBundle warn that using UserManager as a UserProvider is deprecated and wrong!

Author: rcousens

Resources/doc/index.md

@@ -566,7 +566,7 @@ fos_oauth_server:
     ...
 
     service:
-        user_provider: fos_user.user_manager
+        user_provider: fos_user.user_provider.username
 ```
 
 ## Creating A Client