Update service_container/factories.rst

Author: OskarStark

service_container/factories.rst

@@ -138,6 +138,7 @@ called to create its object. In this case we don't need a dedicated factory clas
             App\Email\NewsletterManager:
                 # the first argument is the class and the second argument is the static method
                 # note that we omit the '@' prefix when using the class name as the first argument!
+                # you can also remove the class name entirely and use 'null' instead
                 factory: ['App\Email\NewsletterManager', 'create']
                 arguments:
                     $sender: 'fabien@symfony.com'