Fix "DateTimeInterface" URL

Change generated URL "http://api.symfony.com/3.4/DateTimeInterface.html" to "http://php.net/manual/en/class.datetimeinterface.php"

serializer.rst

@@ -99,7 +99,7 @@ As well as the following normalizers:
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\ObjectNormalizer` to
   handle typical data objects
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\DateTimeNormalizer` for
-  objects implementing the :class:`DateTimeInterface` interface
+  objects implementing the `DateTimeInterface`_ interface
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\DataUriNormalizer` to
   transform :class:`SplFileInfo` objects in `Data URIs`_
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\JsonSerializableNormalizer`
@@ -332,4 +332,5 @@ take a look at how this bundle works.
 .. _`ApiPlatform`: https://github.com/api-platform/core
 .. _`JSON-LD`: http://json-ld.org
 .. _`Hydra Core Vocabulary`: http://hydra-cg.com
+.. _`DateTimeInterface`: http://php.net/manual/en/class.datetimeinterface.php
 .. _`Data URIs`: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs