minor #9534 Fix "DateTimeInterface" URL (liviubalan, Liviu Balan)

This PR was merged into the 3.4 branch.

Discussion
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Fix "DateTimeInterface" URL

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

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Commits
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b99957b Changes after code review
1ebd63d Fix "DateTimeInterface" URL

Author: javiereguiluz

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 :phpclass:`DateTimeInterface` interface
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\DataUriNormalizer` to
   transform :class:`SplFileInfo` objects in `Data URIs`_
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\JsonSerializableNormalizer`