Invalid download URL of WMT17 `zh-en` data 
## Describe the bug
Partial data (wmt17 zh-en) cannot be downloaded due to an invalid URL.

## Steps to reproduce the bug
```python
from datasets import load_dataset
 dataset = load_dataset('wmt17','zh-en')
```

## Expected results
ConnectionError: Couldn't reach ftp://cwmt-wmt:cwmt-wmt@datasets.nju.edu.cn/parallel/casia2015.zip