minor #19743 [Serializer] Fix punctuation in custom encoders docs (mttsch)

This PR was merged into the 5.4 branch.

Discussion
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[Serializer] Fix punctuation in custom encoders docs

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Commits
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815e8de [Serializer] Fix punctuation in custom encoders docs

Author: javiereguiluz

serializer/custom_encoders.rst

@@ -56,7 +56,7 @@ create your own encoder that uses the
 Registering it in your app
 --------------------------
 
-If you use the Symfony Framework. then you probably want to register this encoder
+If you use the Symfony Framework, then you probably want to register this encoder
 as a service in your app. If you're using the :ref:`default services.yaml configuration <service-container-services-load-example>`,
 that's done automatically!