Error while building documentation
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After some investigation, maybe the bug is in `doc-builder`.

I've opened an issue there:
- huggingface/doc-builder#160
2022-03-28T09:24:11Z
https://github.com/huggingface/datasets/issues/4037#issuecomment-1080405155
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MEMBER
Fixed by @lewtun (thank you):
- huggingface/doc-builder@31fe6c8bc7225810e281c2f6c6cd32f38828c504
2022-03-28T10:00:48Z
https://github.com/huggingface/datasets/issues/4037#issuecomment-1080448340
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2022-03-28T10:01:52Z
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## Describe the bug
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.
```

