Fix grammar in asyncio-dev.rst (GH-15672)

Automerge-Triggered-By: @ned-deily

Doc/library/asyncio-dev.rst

@@ -119,7 +119,7 @@ all concurrent asyncio Tasks and IO operations would be delayed
 by 1 second.
 
 An executor can be used to run a task in a different thread or even in
-a different process to avoid blocking block the OS thread with the
+a different process to avoid blocking the OS thread with the
 event loop.  See the :meth:`loop.run_in_executor` method for more
 details.