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

Fix #3626.