load dataset wikitext-2-raw-v1 failed. Could not reach wikitext-2-raw-v1.py.
## Describe the bug
Could not reach wikitext-2-raw-v1.py

## Steps to reproduce the bug
```python
from datasets import load_dataset
load_dataset("wikitext-2-raw-v1")
```

## Expected results
Download `wikitext-2-raw-v1` dataset successfully.

## Actual results
```
  File "load_datasets.py", line 13, in <module>
    load_dataset("wikitext-2-raw-v1")
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 1715, in load_dataset
    **config_kwargs,
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 1536, in load_dataset_builder
    data_files=data_files,
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 1282, in dataset_module_factory
    raise e1 from None
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 1224, in dataset_module_factory
    dynamic_modules_path=dynamic_modules_path,
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 559, in get_module
    local_path = self.download_loading_script(revision)
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/load.py", line 539, in download_loading_script
    return cached_path(file_path, download_config=download_config)
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/utils/file_utils.py", line 246, in cached_path
    download_desc=download_config.download_desc,
  File "/root/miniconda3/lib/python3.6/site-packages/datasets/utils/file_utils.py", line 582, in get_from_cache
    raise ConnectionError(f"Couldn't reach {url} ({repr(head_error)})")
ConnectionError: Couldn't reach https://raw.githubusercontent.com/huggingface/datasets/2.2.2/datasets/wikitext-2-raw-v1/wikitext-2-raw-v1.py (ReadTimeout(ReadTimeoutError("HTTPSConnectionPool(host='raw.githubusercontent.com', port=443): Read timed out. (read timeout=100)",),))
```
I tried to download wikitext-2-raw-v1.py by chrome and got:
![image](https://user-images.githubusercontent.com/20658907/170171595-0ca9f1da-c05a-4b57-861e-9530bfa3bdb9.png)

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.2.2
- Platform: CentOS 7
- Python version: 3.6
- PyArrow version: 3.0.0
