update mirror for RT dataset
Thanks for adding this mirror link :)

Could you run the following command to update the json file `dataset_infos.json` used to verify the integrity of the downloaded file ?

```
nlp-cli test ./datasets/rotten_tomatoes --save_infos --ignore_verifications
``` done! @lhoestq  the build_doc CI fail comes from master and has been fixed on master done @thomwolf @lhoestq 