Fix Sphinx markup for asyncio modules so :mod:`asyncio` links work.

Doc/library/asyncio-eventloop.rst

-.. module:: asyncio
+.. currentmodule:: asyncio
 
 .. _event-loop:
 

Doc/library/asyncio-protocol.rst

-.. module:: asyncio
+.. currentmodule:: asyncio
 
 ++++++++++++++++++++++++
 Transports and protocols

Doc/library/asyncio-sync.rst

+.. currentmodule:: asyncio
 .. _sync:
 
 Synchronization primitives

Doc/library/asyncio-task.rst

-.. module:: asyncio
+.. currentmodule:: asyncio
 
 Tasks and coroutines
 ====================