change API documentation from time.core to just time

This is a pretty trivial PR that probably only @taldcroft will care about.

This changes the documentation for the `time` subpackage to be an API for `astropy.time` instead of `astropy.time.core`.  Right now, this doesn't change what classes/functions are documented because ``astropy/time/__init__.py`` just has ``from .core import *`` in it.  

I think it's better to do it this way, though, because now if I put `` `astropy.time` `` in the docs (or someone else does via intersphinx) it will link to that page, instead of having no link (because it thinks it's documentiong `astropy.time.core`).  It's also marginally clearer this way to users that they should do ``from astropy.time import foo`` instead of ``from astropy.time.core import foo``.

Or is there some other reason you had for using `core` over the base package, @taldcroft?