Fix commit message formatting in multi-commit uploads
Currently, the commit message keeps on adding:

- `Upload dataset (part 00000-of-00002)`
- `Upload dataset (part 00000-of-00002) (part 00001-of-00002)`

Introduced in https://github.com/huggingface/datasets/pull/6269

This PR fixes this issue to have

- `Upload dataset (part 00000-of-00002)`
- `Upload dataset (part 00001-of-00002)`