minor #17736 [Form] Add missing imports in unmapped fields example (alexandre-daubois)

This PR was merged into the 5.4 branch.

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[Form] Add missing imports in unmapped fields example

Fixes #17698

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57fae2a [Form] Add missing imports in unmapped fields example

Author: javiereguiluz

forms.rst

@@ -968,6 +968,8 @@ example to add an *"I agree with these terms"* checkbox), set the ``mapped``
 option to ``false`` in those fields::
 
     // ...
+    use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
+    use Symfony\Component\Form\Extension\Core\Type\SubmitType;
     use Symfony\Component\Form\FormBuilderInterface;
 
     class TaskType extends AbstractType