Fixed allow_migrate() signature in router examples.

docs/topics/db/multi-db.txt

@@ -314,7 +314,7 @@ send queries for the ``auth`` app to ``auth_db``::
                return True
             return None
 
-        def allow_migrate(self, db, app_label, model, **hints):
+        def allow_migrate(self, db, app_label, model=None, **hints):
             """
             Make sure the auth app only appears in the 'auth_db'
             database.
@@ -352,7 +352,7 @@ from::
                 return True
             return None
 
-        def allow_migrate(self, db, app_label, model, **hints):
+        def allow_migrate(self, db, app_label, model=None, **hints):
             """
             All non-auth models end up in this pool.
             """