bpo-33649: Add low-level APIs index. (GH-9364)

diff --git a/Doc/library/asyncio-future.rst b/Doc/library/asyncio-future.rst
index 19bf8a6..d6c5335 100644
--- a/Doc/library/asyncio-future.rst
+++ b/Doc/library/asyncio-future.rst
@@ -191,6 +191,8 @@
       .. versionadded:: 3.7
 
 
+.. _asyncio_example_future:
+
 This example creates a Future object, creates and schedules an
 asynchronous Task to set result for the Future, and waits until
 the Future has a result::