bpo-33649: Fix markup; add another note that asyncio.run is 3.7+ (GH-9389)

diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst
index bfc9700..1511b2f 100644
--- a/Doc/library/asyncio.rst
+++ b/Doc/library/asyncio.rst
@@ -8,7 +8,7 @@
 
 .. sidebar:: Hello World!
 
-   .. code-block:: python
+   ::
 
        import asyncio