Add the link to asyncio source code in the docs (GH-2373)

diff --git a/Doc/library/asyncio-sync.rst b/Doc/library/asyncio-sync.rst
index 0909352..14e3def 100644
--- a/Doc/library/asyncio-sync.rst
+++ b/Doc/library/asyncio-sync.rst
@@ -4,6 +4,8 @@
 Synchronization primitives
 ==========================
 
+**Source code:** :source:`Lib/asyncio/locks.py`
+
 Locks:
 
 * :class:`Lock`