Fix typo in docstring for O365 connector card action (#2244)

https://github.com/microsoft/botbuilder-python/blob/main/libraries/botbuilder-core/botbuilder/core/teams/teams_activity_handler.py
#PingMSFTDocs

Author: hyoshioka0128

libraries/botbuilder-core/botbuilder/core/teams/teams_activity_handler.py

@@ -314,7 +314,7 @@ async def on_teams_o365_connector_card_action(  # pylint: disable=unused-argumen
         self, turn_context: TurnContext, query: O365ConnectorCardActionQuery
     ):
         """
-        Invoked when a O365 Connector Card Action activity is received from the connector.
+        Invoked when an O365 Connector Card Action activity is received from the connector.
 
         :param turn_context: A context object for this turn.
         :param query: The O365 connector card HttpPOST invoke query.