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

Fix #3626.