Croissant: fix sha256 field
We currently return:

```
"sha256": "https://github.com/mlcommons/croissant/issues/80"
```

See https://github.com/mlcommons/croissant/issues/80. cc @marcenacp 
Croissant: fix sha256 field: We currently return:

```
"sha256": "https://github.com/mlcommons/croissant/issues/80"
```

See https://github.com/mlcommons/croissant/issues/80. cc @marcenacp 