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

(cherry picked from commit b042cf10c6084d14279c55a7e0d2d7595ff4e694)

Co-authored-by: Yury Selivanov <yury@magic.io>
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