Fix a typo in asyncio-dev.rst. (GH-10133)

"threadsafe"
(cherry picked from commit 4e3a53bc)
Co-authored-by: Benjamin Peterson <benjamin@python.org>

Doc/library/asyncio-dev.rst

@@ -50,7 +50,7 @@ When the debug mode is enabled:
   <asyncio-coroutine-not-scheduled>` and logs them; this mitigates
   the "forgotten await" pitfall.
 
-* Many non-treadsafe asyncio APIs (such as :meth:`loop.call_soon` and
+* Many non-threadsafe asyncio APIs (such as :meth:`loop.call_soon` and
   :meth:`loop.call_at` methods) raise an exception if they are called
   from a wrong thread.