Update .readthedocs.org links to point to readthedocs.io
Update .readthedocs.org links to point to readthedocs.io

Read the Docs sent out an email saying they were updating their site hosting
to readthedocs.io instead of readthedocs.org. Though the old link redirects to
the new location automatically, update the link to point to the right spot in
the first place.

chapel-lang/chplforpyp-docs