Fix docs settings default value

Author: rpatterson

docs/configuration.rst

@@ -36,7 +36,7 @@ Configuration
     You can define your custom serializers for registration endpoint.
     Possible key values:
 
-        - REGISTER_SERIALIZER - serializer class in ``rest_auth.register.views.RegisterView``, default value ``rest_auth.register.serializers.RegisterSerializer``
+        - REGISTER_SERIALIZER - serializer class in ``rest_auth.register.views.RegisterView``, default value ``rest_auth.registration.serializers.RegisterSerializer``
 
 - **REST_AUTH_TOKEN_MODEL** - model class for tokens, default value ``rest_framework.authtoken.models``