minor #965 Update serializer.rst (ruslan-polutsygan)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #965).

Discussion
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Update serializer.rst

Before - http://i.imgur.com/hTYzHGR.png
After - http://i.imgur.com/SAPhNN7.png

Commits
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a8ab1f8b Update serializer.rst

doc/providers/serializer.rst

@@ -11,17 +11,17 @@ None.
 Services
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-* **serializer**: An instance of `Symfony\Component\Serializer\Serializer
+* **serializer**: An instance of `Symfony\\Component\\Serializer\\Serializer
   <http://api.symfony.com/master/Symfony/Component/Serializer/Serializer.html>`_.
 
-* **serializer.encoders**: `Symfony\Component\Serializer\Encoder\JsonEncoder
+* **serializer.encoders**: `Symfony\\Component\\Serializer\\Encoder\\JsonEncoder
   <http://api.symfony.com/master/Symfony/Component/Serializer/Encoder/JsonEncoder.html>`_
-  and `Symfony\Component\Serializer\Encoder\XmlEncoder
+  and `Symfony\\Component\\Serializer\\Encoder\\XmlEncoder
   <http://api.symfony.com/master/Symfony/Component/Serializer/Encoder/XmlEncoder.html>`_.
 
-* **serializer.normalizers**: `Symfony\Component\Serializer\Normalizer\CustomNormalizer
+* **serializer.normalizers**: `Symfony\\Component\\Serializer\\Normalizer\\CustomNormalizer
   <http://api.symfony.com/master/Symfony/Component/Serializer/Normalizer/CustomNormalizer.html>`_
-  and `Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer
+  and `Symfony\\Component\\Serializer\\Normalizer\\GetSetMethodNormalizer
   <http://api.symfony.com/master/Symfony/Component/Serializer/Normalizer/GetSetMethodNormalizer.html>`_.
 
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