minor #19979 [Serializer] Update serializer.rst (novah77)

This PR was submitted for the 7.2 branch but it was merged into the 6.4 branch instead.

Discussion
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[Serializer] Update serializer.rst

In the sub title Attributes Groups, the text using annotations should be using attributes.

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Commits
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a84f466 Update serializer.rst

Author: javiereguiluz

components/serializer.rst

@@ -264,7 +264,7 @@ Assume you have the following plain-old-PHP object::
         }
     }
 
-The definition of serialization can be specified using annotations, XML
+The definition of serialization can be specified using attributes, XML
 or YAML. The :class:`Symfony\\Component\\Serializer\\Mapping\\Factory\\ClassMetadataFactory`
 that will be used by the normalizer must be aware of the format to use.