Fixed a reference

Author: javiereguiluz

doctrine/registration_form.rst

@@ -13,5 +13,5 @@ form you must:
 #. :ref:`Define a class to represent users <create-user-class>`;
 #. :doc:`Create a form </forms>` to ask for the registration information;
 #. Create :doc:`a controller </controller>` to :doc:`process the form <processing-forms>`;
-#. :doc:`Protect some parts of your application <security-authorization>` so
+#. :ref:`Protect some parts of your application <security-authorization>` so
    only registered users can access to them.