asyncio: Fix docs for default event loop (#14308)

When the Windows default event loop changed, `asyncio-policy.rst` was updated but `asyncio-eventloop.rst` was missed.

Doc/library/asyncio-eventloop.rst

@@ -1454,7 +1454,7 @@ asyncio ships with two different event loop implementations:
 :class:`SelectorEventLoop` and :class:`ProactorEventLoop`.
 
 By default asyncio is configured to use :class:`SelectorEventLoop`
-on all platforms.
+on Unix and :class:`ProactorEventLoop` on Windows.
 
 
 .. class:: SelectorEventLoop