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-task.rst b/Doc/library/asyncio-task.rst
index 2753998..45b8b60 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -20,7 +20,8 @@
 
 Coroutines declared with async/await syntax is the preferred way of
 writing asyncio applications.  For example, the following snippet
-of code prints "hello", waits 1 second, and then prints "world"::
+of code (requires Python 3.7+) prints "hello", waits 1 second,
+and then prints "world"::
 
     >>> import asyncio