Fix comment

Author: AnatoliB

azure/durable_functions/models/DurableOrchestrationContext.py

@@ -775,7 +775,7 @@ def version(self) -> str:
 
     @staticmethod
     def _extract_version_from_history(history_events: List[HistoryEvent]) -> str:
-        """Extract the version from the execution started event in history, or None if not found"""
+        """Extract the version from the execution started event in history, or None if not found."""
         for event in history_events:
             if event.event_type == HistoryEventType.EXECUTION_STARTED:
                 return event.Version