Fix missing coroutine declaration in the asyncio documentation. (GH-5964)

(cherry picked from commit 13cfd57d)
Co-authored-by: Joongi Kim <me@daybreaker.info>

Doc/library/asyncio-task.rst

@@ -630,7 +630,7 @@ Task functions
 
    This function is a :ref:`coroutine <coroutine>`.
 
-.. function:: shield(arg, \*, loop=None)
+.. coroutinefunction:: shield(arg, \*, loop=None)
 
    Wait for a future, shielding it from cancellation.