minor #6000 Update registration_form.rst (afurculita)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6000).

Discussion
----------

Update registration_form.rst

Commits
-------

cac8c2d Update registration_form.rst

Author: wouterj

cookbook/doctrine/registration_form.rst

@@ -199,7 +199,7 @@ Handling the Form Submission
 Next, you need a controller to handle the form. Start by creating a simple
 controller for displaying the registration form::
 
-    // src/AppBundle/Controller/AccountController.php
+    // src/AppBundle/Controller/RegistrationController.php
     namespace AppBundle\Controller;
 
     use Symfony\Bundle\FrameworkBundle\Controller\Controller;