I'll make a simple fix for the asyncio.coroutine decorator docs.

> I would suggest below for #coroutines :  

>> Coroutines declared with async/await syntax is the preferred way of writing asyncio applications but asyncio also supports legacy generator-based coroutines.

I don't want to further emphasize generator-based coroutines beyond how they are covered right now.  We'll drop support for them in a just a couple of versions anyways.