Make it more obvious what the $handler is (refers to the next item in the docs)

closes #13148

Author: jpjoao

components/messenger.rst

@@ -89,10 +89,13 @@ are configured for you:
 Example::
 
     use App\Message\MyMessage;
+    use App\MessageHandler\MyMessageHandler;
     use Symfony\Component\Messenger\Handler\HandlersLocator;
     use Symfony\Component\Messenger\MessageBus;
     use Symfony\Component\Messenger\Middleware\HandleMessageMiddleware;
 
+    $handler = new MyMessageHandler();
+
     $bus = new MessageBus([
         new HandleMessageMiddleware(new HandlersLocator([
             MyMessage::class => [$handler],