minor #10200 action_method is missing $task variable (TomasVotruba)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes #10200).

Discussion
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action_method is missing $task variable

It's not clear from the docs, but user has to first create the `$task` variable.

In the second example bellow it's more clear thanks to `// ...`.

This makes it a bit more explicit here as well.

Commits
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5ada0ef action_method is missing $task variable

Author: xabbuh

form/action_method.rst

@@ -27,6 +27,8 @@ form, you can use ``setAction()`` and ``setMethod()``:
         {
             public function newAction()
             {
+                // ...
+                
                 $form = $this->createFormBuilder($task)
                     ->setAction($this->generateUrl('target_route'))
                     ->setMethod('GET')