[Messenger] Fix instance variable declaration

Author: MaximePinot

messenger.rst

@@ -2548,7 +2548,7 @@ using the ``DispatchAfterCurrentBusMiddleware`` and adding a
     {
         public function __construct(
             private MailerInterface $mailer,
-            EntityManagerInterface $em,
+            private EntityManagerInterface $em,
         ) {
         }