Can't load dataset
Hi @mskovalova, 

Some datasets have multiple configurations. Therefore, in order to load them, you have to specify both the *dataset name* and the *configuration name*.

In the error message you got, you have a usage example:
- To load the 'wikitext-103-raw-v1' configuration of the 'wikitext' dataset, you should use: 
  ```python
  load_dataset('wikitext', 'wikitext-103-raw-v1')
  ```

In your case, if you would like to load the 'wikitext-2-v1' configuration of the 'wikitext' dataset, please use:
```python
raw_datasets = load_dataset("wikitext", "wikitext-2-v1")
```
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
