minor #13601 [Error pages] Add missing use (jeremyFreeAgent)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
----------

[Error pages] Add missing use

Commits
-------

c76ae44 [Error pages] Add missing use

Author: javiereguiluz

controller/error_pages.rst

@@ -212,6 +212,7 @@ contents, create a new Normalizer that supports the ``FlattenException`` input::
     # src/App/Serializer/MyCustomProblemNormalizer.php
     namespace App\Serializer;
 
+    use Symfony\Component\ErrorHandler\Exception\FlattenException;
     use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
 
     class MyCustomProblemNormalizer implements NormalizerInterface