minor #18798 [Serializer] Fix Serializer context configuration in YAML (wuchen90)

This PR was merged into the 5.4 branch.

Discussion
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[Serializer] Fix Serializer context configuration in YAML

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See the code of v5.4:

https://github.com/symfony/symfony/blob/fcb754a1c08ef0b283f1f0caf0c8666098cb7fb6/src/Symfony/Component/Serializer/Mapping/Loader/YamlFileLoader.php#L105-L119

Commits
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dc67e90 Fix Serializer context configuration in YAML

Author: javiereguiluz

serializer.rst

@@ -212,8 +212,8 @@ You can also specify the context on a per-property basis::
         App\Model\Person:
             attributes:
                 createdAt:
-                    context:
-                        datetime_format: 'Y-m-d'
+                    contexts:
+                        - { context: { datetime_format: 'Y-m-d' } }
 
     .. code-block:: xml