[typo] invalid URL
In <https://github.com/huggingface/transformers/blob/master/docs/source/training.rst?plain=1#L36>

```
We will use the `🤗 Datasets <https:/github.com/huggingface/datasets/>`__ library to download and preprocess the IMDB
```

Correct URL should be <https://github.com/huggingface/datasets/>.
Closed by #13644