minor #5866 Fixed little typo with a twig example (artf)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5866).

Discussion
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Fixed little typo with a twig example

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.7
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....
```

Commits
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758e5c9 Fixed little typo with a twig example

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) }}