minor #12943 [Form] Use the get method instead of using the array (Arman-Hosseini)

This PR was merged into the 3.4 branch.

Discussion
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[Form] Use the get method instead of using the array

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Commits
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0da5901 Use the get method instead of using the array

Author: OskarStark

controller/upload_file.rst

@@ -143,7 +143,7 @@ Finally, you need to update the code of the controller that handles the form::
 
             if ($form->isSubmitted() && $form->isValid()) {
                 /** @var UploadedFile $brochureFile */
-                $brochureFile = $form['brochure']->getData();
+                $brochureFile = $form->get('brochure')->getData();
 
                 // this condition is needed because the 'brochure' field is not required
                 // so the PDF file must be processed only when a file is uploaded