Fixed a mistake in the docs about form creation

(you cannot simply call "->createForm()" in "any service that has access to the form factory")

Author: Burgov

cookbook/form/dynamic_form_modification.rst

@@ -370,16 +370,22 @@ it with :ref:`dic-tags-form-type`.
 If you wish to create it from within a controller or any other service that has
 access to the form factory, you then use::
 
-    class FriendMessageController extends Controller
+    use Symfony\Component\DependencyInjection\ContainerAware;
+
+    class FriendMessageController extends ContainerAware
     {
         public function newAction(Request $request)
         {
-            $form = $this->createForm('acme_friend_message');
+            $form = $this->get('form.factory')->create('acme_friend_message');
 
             // ...
         }
     }
 
+If you extend the ``Symfony\Bundle\FrameworkBundle\Controller\Controller`` class, you can simply call::
+
+    $form = $this->createForm('acme_friend_message');
+
 You can also easily embed the form type into another form::
 
     // inside some other "form type" class