Fix typo in Custom XCom backend (#10588)

(cherry picked from commit 2fc9614)

docs/concepts.rst

@@ -663,7 +663,7 @@ for inter-task communication rather than global settings.
 Custom XCom backend
 '''''''''''''''''''
 
-It is possible to change ``XCom`` behaviour os serialization and deserialization of tasks' result.
+It is possible to change ``XCom`` behaviour of serialization and deserialization of tasks' result.
 To do this one have to change ``xcom_backend`` parameter in Airflow config. Provided value should point
 to a class that is subclass of :class:`~airflow.models.xcom.BaseXCom`. To alter the serialaization /
 deserialization mechanism the custom class should override ``serialize_value`` and ``deserialize_value``