Fix incorrect MRQA train+SQuAD URL
Thanks ! could you regenerate the dataset_infos.json file ?

```
datasets-cli test ./datasets/mrqa --save_infos --all_configs --ignore_verifications
```

also cc @VictorSanh  Oooops, good catch @jimmycode  > Thanks ! could you regenerate the dataset_infos.json file ?
> 
> ```
> datasets-cli test ./datasets/mrqa --save_infos --all_configs --ignore_verifications
> ```
> 
> also cc @VictorSanh

Updated the `dataset_infos.json` file.
Fix issue #1115 
