Fix a typo in asyncio docs (GH-5721)

(cherry picked from commit 17ab8f0e8e2b2f66d11fbb95aa9936544a5e1a9a)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>
diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index a19c670..3ee9939 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -1043,7 +1043,7 @@
           async def main(host, port):
               srv = await asyncio.start_server(
                   client_connected, host, port)
-              await loop.serve_forever()
+              await srv.serve_forever()
 
           asyncio.run(main('127.0.0.1', 0))