Fix checksum verification
Expected checksum was verified against checksum dict (not checksum).
Hi ! If I'm not mistaken both `expected_checksums[url]` and `recorded_checksums[url]` are dictionaries with keys "checksum" and "num_bytes". So we need to check whether `expected_checksums[url] != recorded_checksums[url]` (or simply `expected_checksums[url]["checksum"] != recorded_checksums[url]["checksum"]`)

But in your fix you're checking `expected_checksums[url] != recorded_checksums[url]['checksum']`.

So I think it's fine to keep this as is No, the issue is that there is comparison of sclar value and dictionary. Acording to [`DatasetInfo`][1], we need specify a dictionary which maps a URL to a checksum as follows.

```python
CHECKSUMS = {
    URL: 'a5dc6bf63ea088ade6e98594bfa386f45211c38b2a3db3dd11b33bd530f3c481',
}

class FancyDataset:
    def _info(self):
        return DatasetInfo(..., download_checksums=CHECKSUMS)
```

However, `load_dataset` fails with this checksum definition.

[1]: https://github.com/huggingface/datasets/blob/main/src/datasets/info.py#L124-L125 I think it has to be formatted like this right now. Maybe the DatasetInfo doc is unclear and we can improve it
```python
CHECKSUMS = {
    URL: {"checksum": checksum, "num_bytes": num_bytes},
}
``` Right. I am not sure that this is a correct way to do it. People usually calculate sha256, md5, or whatever else but not size in bytes. Also, people use only some of checksum algorithms. This means that comparing dictionaries in `verify_checksums` is too strict (requires equality of all items) and raises compatibility issues in the future. Another issue is that a comparison of dictionaries assumes type constraints which imply type equality. 

Since almost noone uses checksums as far as I known, my PR suggests a minimal change to mitigate these issues except support of a specific checksum algorithm which is a separated feature and should be contributed in a separate PRs from my perspective. Applying this change will break the verification code, since the `expected_checksums` is a dict with those two keys. The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5396). All of your documentation changes will be reflected on that endpoint.