OpenSLR/83 is empty
Hi @tyrius02, thanks for reporting. I see you self-assigned this issue: are you working on this? @albertvillanova Yes. Figured I introduced the broken config, I should fix it too.

I've got it working, but I'm struggling with one of the tests. I've started a PR so I/we can work through it. Looks like the tests all passed on the PR.
## Describe the bug
As the summary says, openslr / SLR83 / train is empty.

The dataset returned after loading indicates there are **zero** rows. The correct number should be **17877**.
## Steps to reproduce the bug
```python
import datasets


datasets.load_dataset('openslr', 'SLR83')
```

## Expected results
```
DatasetDict({
    train: Dataset({
        features: ['path', 'audio', 'sentence'],
        num_rows: 17877
    })
})
```
## Actual results
```
DatasetDict({
    train: Dataset({
        features: ['path', 'audio', 'sentence'],
        num_rows: 0
    })
})
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.14.1.dev0 (master HEAD)
- Platform: Ubuntu 20.04
- Python version: 3.7.10
- PyArrow version: 3.0.0
