[Serializer] Fix punctuation in custom encoders docs

Author: mttsch

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!