Fixed little typo with a twig example

Was stuck with this throw:
 Twig_Error_Runtime: An exception has been thrown during the rendering of a template ("Catchable Fatal Error: Argument 1 passed to Symfony\Component\Form\FormRenderer::searchAndRenderBlock() must be an instance of Symfony\Component\Form\FormView, string given, called in....

Author: artf

cookbook/doctrine/registration_form.rst

@@ -298,10 +298,10 @@ Next, create the template:
         {# app/Resources/views/registration/register.html.twig #}
 
         {{ form_start(form) }}
-            {{ form_row('form.username') }}
-            {{ form_row('form.email') }}
-            {{ form_row('form.plainPassword.first') }}
-            {{ form_row('form.plainPassword.second') }}
+            {{ form_row(form.username) }}
+            {{ form_row(form.email) }}
+            {{ form_row(form.plainPassword.first) }}
+            {{ form_row(form.plainPassword.second) }}
 
             <button type="submit">Register!</button>
         {{ form_end(form) }}