minor symfony#6138 Correction needed (pfleu)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes symfony#6138).

Discussion
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Correction needed

There is an error in the prototype customization chapter. "tasks" must be "form"

Commits
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87d6884 Correction needed

Author: xabbuh

cookbook/form/form_customization.rst

@@ -762,8 +762,8 @@ will be able to change the widget for each task as follows:
 
         {% block _tasks_entry_widget %}
             <tr>
-                <td>{{ form_widget(task.task) }}</td>
-                <td>{{ form_widget(task.dueDate) }}</td>
+                <td>{{ form_widget(form.task) }}</td>
+                <td>{{ form_widget(form.dueDate) }}</td>
             </tr>
         {% endblock %}