Fix host URL in The Pile datasets
COLLABORATOR
We should also update the `bookcorpusopen` download url (see #3561) , no? 
2022-01-25T14:26:24Z
https://github.com/huggingface/datasets/pull/3627#issuecomment-1021235732
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2022-01-25T14:26:24Z
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MEMBER
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. 
2022-01-26T09:12:25Z
https://github.com/huggingface/datasets/pull/3627#issuecomment-1022004520
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2022-01-26T09:12:39Z
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NONE
Seems like the mystic URL is now broken and the original should be used. 
2022-07-20T18:56:57Z
https://github.com/huggingface/datasets/pull/3627#issuecomment-1190641040
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2022-07-20T18:56:57Z
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NONE
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? 
2022-07-20T20:54:42Z
https://github.com/huggingface/datasets/pull/3627#issuecomment-1190748170
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This PR fixes the host URL in The Pile datasets, once they have mirrored their data in another server.

Fix #3626.