minor #4969 Add typehint (piotrantosik)

This PR was merged into the 2.3 branch.

Discussion
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Add typehint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets | -

Commits
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7e7ca06 add typehint

cookbook/form/dynamic_form_modification.rst

@@ -721,10 +721,11 @@ all of this, use a listener::
 
     use Symfony\Component\Form\FormBuilderInterface;
     use Symfony\Component\Form\FormEvents;
+    use Symfony\Component\Form\FormEvent;
 
     public function buildForm(FormBuilderInterface $builder, array $options)
     {
-        $builder->addEventListener(FormEvents::POST_SUBMIT, function ($event) {
+        $builder->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
             $event->stopPropagation();
         }, 900); // Always set a higher priority than ValidationListener