Fix double sequence
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7672). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.
```python
>>> Features({"a": Sequence(Sequence({"c": Value("int64")}))})
{'a': List({'c': List(Value('int64'))})}
```

instead of `{'a': {'c': List(List(Value('int64')))}}`