expanduser in save_to_disk
### Describe the bug

save_to_disk() does not expand `~`

1. `dataset = load_datasets("any dataset")`
2. `dataset.save_to_disk("~/data")`
3. a folder named "~" created in current folder
4. FileNotFoundError is raised, because the expanded path does not exist (`/home/<user>/data`)

related issue https://github.com/huggingface/transformers/issues/10628

### Steps to reproduce the bug

As described above.

### Expected behavior

expanduser correctly

### Environment info

- datasets 2.10.1
- python 3.10