Dataset viewer issue for 'liweili/c4_200m'
Hi ! I think the issue comes from this [line](https://huggingface.co/datasets/liweili/c4_200m/blob/main/c4_200m.py#L87):
```python
path = filepath + "/*.tsv*"
```

You can fix this by doing this instead:
```python
path = os.path.join(filepath, "/*.tsv*")
```

Here is why:

Locally you can append `"/*.tsv*"` to your local path, however it doesn't work in streaming mode, and the dataset viewer does use the streaming mode.
In streaming mode, the download and extract part is done lazily. It means that instead of using local paths, it's still passing around URLs and [chained URLs](https://filesystem-spec.readthedocs.io/en/latest/features.html#url-chaining)

Therefore in streaming mode, `filepath` is not a local path, but instead is equal to
```python
zip://::https://huggingface.co/datasets/liweili/c4_200m/resolve/main/data.zip
```
The `zip://` part means that we navigate inside the remote ZIP file.

You must use `os.path.join` to navigate inside it and get your TSV files:
```python
>>> os.path.join(filepath, "/*.tsv*")
zip://*.tsv*::https://huggingface.co/datasets/liweili/c4_200m/resolve/main/data.zip
```

`datasets` extends `os.path.join`, `glob.glob`, etc. in your dataset scripts to work with remote files. hi @lhoestq ! thanks for the tip! i've updated the line of code but it's still not working. am i doing something else wrong? thank you! Hi ! Your dataset code is all good now :)
```python
In [1]: from datasets import load_dataset

In [2]: d = load_dataset("liweili/c4_200m", streaming=True)
Downloading: 100%|█████████████████████████████████████████████| 2.79k/2.79k [00:00<00:00, 4.83MB/s]
Using custom data configuration default

In [3]: next(iter(d["train"]))
Out[3]: 
{'input': 'Bitcoin is for $7,094 this morning, which CoinDesk says.',
 'output': 'Bitcoin goes for $7,094 this morning, according to CoinDesk.'}
```
Though the viewer doesn't seem to be updated, I'll take a look at what's wrong thank you @lhoestq! 😄  It's working

<img width="1424" alt="Capture d’écran 2021-12-21 à 11 24 29" src="https://user-images.githubusercontent.com/1676121/146914238-24bf87c0-c68d-4699-8d6c-fa3065656d1d.png">


## Dataset viewer issue for '*liweili/c4_200m*'

**Link:** *[link to the dataset viewer page](https://huggingface.co/datasets/liweili/c4_200m)*

*Server Error*
```
Status code:   404
Exception:     Status404Error
Message:       Not found. Maybe the cache is missing, or maybe the ressource does not exist.
```

Am I the one who added this dataset ? Yes
