Can't load dataset
I'm trying to load a wikitext dataset

```
from datasets import load_dataset
raw_datasets = load_dataset("wikitext")
```

ValueError: Config name is missing.
Please pick one among the available configs: ['wikitext-103-raw-v1', 'wikitext-2-raw-v1', 'wikitext-103-v1', 'wikitext-2-v1']
Example of usage:
	`load_dataset('wikitext', 'wikitext-103-raw-v1')`.

If I try
```
from datasets import load_dataset
raw_datasets = load_dataset("wikitext-2-v1")
```

FileNotFoundError: Couldn't find file at https://raw.githubusercontent.com/huggingface/datasets/1.12.1/datasets/wikitext-2-v1/wikitext-2-v1.py

#### Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.12.1
- Platform: Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic (colab)
- Python version: 3.7.12
- PyArrow version: 3.0.0
