docs: fix typo in code comments

JIRA: CQ-1005

Author: lupko

gooddata-flight-server/gooddata_flight_server/tasks/thread_task_executor.py

@@ -270,7 +270,7 @@ def cancel(self) -> bool:
         """
         Cancels the execution.
 
-        IMPORTANT: task executor most not hold any locks at the time of cancellation.
+        IMPORTANT: task executor must not hold any locks at the time of cancellation.
 
         :return: True if cancel was successful, false if it was not possible
         """