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

diff --git a/Doc/library/asyncio-queue.rst b/Doc/library/asyncio-queue.rst
index f11c09a..ea78755 100644
--- a/Doc/library/asyncio-queue.rst
+++ b/Doc/library/asyncio-queue.rst
@@ -3,6 +3,8 @@
 Queues
 ======
 
+**Source code:** :source:`Lib/asyncio/queues.py`
+
 Queues:
 
 * :class:`Queue`