Fix host URL in The Pile datasets
This PR fixes the host URL in The Pile datasets, once they have mirrored their data in another server.

Fix #3626.
We should also update the `bookcorpusopen` download url (see #3561) , no?  For `the_pile_openwebtext2` and `the_pile_stack_exchange` I did not regenerate the JSON files, but instead I just changed the download_checksums URL.  Seems like the mystic URL is now broken and the original should be used.  Also if I git clone and edit the repo or reset it before this PR it is still trying to pull using mystic? Why is this? 