minor #16002 Update service container rst with missing class (JeroenMoonen)

This PR was submitted for the 6.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Update service container rst with missing class

Add class definition in given service container example

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Commits
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fa05978 Update service container rst with missing class

Author: javiereguiluz

service_container.rst

@@ -125,18 +125,21 @@ inside your controller::
     use Symfony\Component\HttpFoundation\Response;
     use Symfony\Component\Routing\Annotation\Route;
 
-    /**
-     * @Route("/products/new")
-     */
-    public function new(MessageGenerator $messageGenerator): Response
+    class ProductController
     {
-        // thanks to the type-hint, the container will instantiate a
-        // new MessageGenerator and pass it to you!
-        // ...
+        /**
+         * @Route("/products/new")
+         */
+        public function new(MessageGenerator $messageGenerator): Response
+        {
+            // thanks to the type-hint, the container will instantiate a
+            // new MessageGenerator and pass it to you!
+            // ...
 
-        $message = $messageGenerator->getHappyMessage();
-        $this->addFlash('success', $message);
-        // ...
+            $message = $messageGenerator->getHappyMessage();
+            $this->addFlash('success', $message);
+            // ...
+        }
     }
 
 When you ask for the ``MessageGenerator`` service, the container constructs a new