Add example for createNamed Form

The docs are not very clear where the FormFactory comes from and how to use this method

Author: wimme002

forms.rst

@@ -676,7 +676,19 @@ choice is ultimately up to you.
 .. note::
 
     The form name is automatically generated from the type class name. If you want
-    to modify it, use the :method:`Symfony\\Component\\Form\\FormFactoryInterface::createNamed` method.
+    to modify it, use the :method:`Symfony\\Component\\Form\\FormFactoryInterface::createNamed` method::
+
+        // src/AppBundle/Controller/DefaultController.php
+        use AppBundle\Form\TaskType;
+
+        public function newAction()
+        {
+            $task = ...;
+            $form = $this->container->get('form.factory')->createNamed('name', TaskType::class, $task);
+
+            // ...
+        }
+
     You can even suppress the name completely by setting it to an empty string.
 
 Final Thoughts