streaming dataset with concatenating splits raises an error
## Describe the bug
streaming dataset with concatenating splits raises an error

## Steps to reproduce the bug
```python
from datasets import load_dataset

# no error
repo = "nateraw/ade20k-tiny"
dataset = load_dataset(repo, split="train+validation")
```

```python
from datasets import load_dataset

# error
repo = "nateraw/ade20k-tiny"
dataset = load_dataset(repo, split="train+validation", streaming=True)
```

```sh
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
[<ipython-input-4-a6ae02d63899>](https://localhost:8080/#) in <module>()
      3 # error
      4 repo = "nateraw/ade20k-tiny"
----> 5 dataset = load_dataset(repo, split="train+validation", streaming=True)

1 frames
[/usr/local/lib/python3.7/dist-packages/datasets/builder.py](https://localhost:8080/#) in as_streaming_dataset(self, split, base_path)
   1030             splits_generator = splits_generators[split]
   1031         else:
-> 1032             raise ValueError(f"Bad split: {split}. Available splits: {list(splits_generators)}")
   1033 
   1034         # Create a dataset for each of the given splits

ValueError: Bad split: train+validation. Available splits: ['validation', 'train']
```

[Colab](https://colab.research.google.com/drive/1wMj08_0bym9jnGgByib4lsBPu8NCZBG9?usp=sharing)

## Expected results
load successfully or throws an error saying it is not supported.

## Actual results
above

## Environment info
- `datasets` version: 2.4.0
- Platform: Windows-10-10.0.22000-SP0 (windows11 x64)
- Python version: 3.9.13
- PyArrow version: 8.0.0
- Pandas version: 1.4.3

Hi! Only the name of a particular split ("train", "test", ...) is supported as a split pattern if `streaming=True`. We plan to address this limitation soon. Hi, have you addressed this yet? yes, same error occurs.
```python
from datasets import load_dataset

# error
repo = "nateraw/ade20k-tiny"
dataset = load_dataset(repo, split="train+validation", streaming=True)
```

```python
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
[<ipython-input-3-a6ae02d63899>](https://localhost:8080/#) in <cell line: 5>()
      3 # error
      4 repo = "nateraw/ade20k-tiny"
----> 5 dataset = load_dataset(repo, split="train+validation", streaming=True)

1 frames
[/usr/local/lib/python3.10/dist-packages/datasets/builder.py](https://localhost:8080/#) in as_streaming_dataset(self, split, base_path)
   1265             splits_generator = splits_generators[split]
   1266         else:
-> 1267             raise ValueError(f"Bad split: {split}. Available splits: {list(splits_generators)}")
   1268 
   1269         # Create a dataset for each of the given splits

ValueError: Bad split: train+validation. Available splits: ['train', 'validation']
```

google colab, `datasets==2.12.0`
```
- huggingface_hub version: 0.14.1
- Platform: Linux-5.10.147+-x86_64-with-glibc2.31
- Python version: 3.10.11
- Running in iPython ?: No
- Running in notebook ?: No
- Running in Google Colab ?: No
- Token path ?: /root/.cache/huggingface/token
- Has saved token ?: False
- Configured git credential helpers: 
- FastAI: 2.7.12
- Tensorflow: 2.12.0
- Torch: 2.0.0+cu118
- Jinja2: 3.1.2
- Graphviz: 0.20.1
- Pydot: 1.4.2
- Pillow: 8.4.0
- hf_transfer: N/A
- gradio: N/A
- ENDPOINT: https://huggingface.co/
- HUGGINGFACE_HUB_CACHE: /root/.cache/huggingface/hub
- HUGGINGFACE_ASSETS_CACHE: /root/.cache/huggingface/assets
- HF_TOKEN_PATH: /root/.cache/huggingface/token
- HF_HUB_OFFLINE: False
- HF_HUB_DISABLE_TELEMETRY: False
- HF_HUB_DISABLE_PROGRESS_BARS: None
- HF_HUB_DISABLE_SYMLINKS_WARNING: False
- HF_HUB_DISABLE_EXPERIMENTAL_WARNING: False
- HF_HUB_DISABLE_IMPLICIT_TOKEN: False
- HF_HUB_ENABLE_HF_TRANSFER: False
```
