Fix: wikicorpus - fix keys
MEMBER
The CI error is unrelated to this PR

... merging !
2021-09-06T14:07:20Z
https://github.com/huggingface/datasets/pull/2844#issuecomment-913677176
https://api.github.com/repos/huggingface/datasets/issues/2844
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2021-09-06T14:07:20Z
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public
As mentioned in https://github.com/huggingface/datasets/issues/2552, there is a duplicate keys error in `wikicorpus`.

I fixed that by taking into account the file index in the keys