Fix comments

Author: dunglas

components/serializer.rst

@@ -685,6 +685,7 @@ parameter of the normalizer::
 
     use Symfony\Component\PropertyInfo\Extractor\ReflectionExtractor;
     use Symfony\Component\Serializer\Serializer;
+    use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;
     use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
 
     namespace Acme;