Fix building of documentation
Documentation building is failing:
- https://github.com/huggingface/datasets/runs/5716300989?check_suite_focus=true

```
ValueError: There was an error when converting ../datasets/docs/source/package_reference/main_classes.mdx to the MDX format.
Unable to find datasets.filesystems.S3FileSystem in datasets. Make sure the path to that object is correct.
```

Fix #4037.