Fix Sphinx markup for asyncio modules so :mod:`asyncio` links work.
diff --git a/Doc/library/asyncio-sync.rst b/Doc/library/asyncio-sync.rst
index aaa8b93..3ea49b3 100644
--- a/Doc/library/asyncio-sync.rst
+++ b/Doc/library/asyncio-sync.rst
@@ -1,3 +1,4 @@
+.. currentmodule:: asyncio
 .. _sync:
 
 Synchronization primitives