slice split while streaming
### Describe the bug

When using the `load_dataset` function with streaming set to True, slicing splits is apparently not supported.

Did I miss this in the documentation?

### Steps to reproduce the bug

`load_dataset("lhoestq/demo1",revision=None, streaming=True, split="train[:3]")`

causes ValueError: Bad split: train[:3]. Available splits: ['train', 'test'] in builder.py, line 1213, in as_streaming_dataset

### Expected behavior

The first 3 entries of the dataset as a stream

### Environment info

- `datasets` version: 2.8.0
- Platform: Windows-10-10.0.19045-SP0
- Python version: 3.10.9
- PyArrow version: 10.0.1
- Pandas version: 1.5.2
