Close #20649: Fix typo in asyncio doc. Patch written by Brett Cannon.

Doc/library/asyncio-dev.rst

@@ -13,7 +13,7 @@ Concurrency and multithreading
 ------------------------------
 
 An event loop runs in a thread and executes all callbacks and tasks in the same
-thread. While a task in running in the event loop, no other task is running in
+thread. While a task is running in the event loop, no other task is running in
 the same thread. But when the task uses ``yield from``, the task is suspended
 and the event loop executes the next task.