gh-108224: Fix asyncio doc inconsistency (#108230)

(Spawning subprocesses does not require the event loop to run in the main thread -- only signal handling does.)

Doc/library/asyncio-dev.rst

@@ -99,7 +99,7 @@ To schedule a coroutine object from a different OS thread, the
  # Wait for the result:
  result = future.result()
 
-To handle signals and to execute subprocesses, the event loop must be
+To handle signals the event loop must be
 run in the main thread.
 
 The :meth:`loop.run_in_executor` method can be used with a