minor #19013 Forgot imports (alexandre-castelain)

This PR was submitted for the 7.0 branch but it was merged into the 6.4 branch instead.

Discussion
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Forgot imports

This is just a small fix for the docs, 2 imports missing.

Commits
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2da09d5 Forgot imports

Author: OskarStark

serializer/custom_normalizer.rst

@@ -19,6 +19,8 @@ to customize the normalized data. To do that, leverage the ``ObjectNormalizer``:
 
     use App\Entity\Topic;
     use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
+    use Symfony\Component\Serializer\Normalizer\NormalizerAwareInterface;
+    use Symfony\Component\Serializer\Normalizer\NormalizerAwareTrait;
     use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
     use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;