Update license to bookcorpus dataset card
The smashwords ToS apply for this dataset, we did the same for https://github.com/huggingface/datasets/pull/3525 _The documentation is not available anymore as the PR was closed or merged._
Not entirely sure, following the links here, but it seems the relevant license is at https://github.com/soskek/bookcorpus/blob/master/LICENSE