sourmash lca database link in tutorial is wrong

Sourmash tutorial link:
https://sourmash.readthedocs.io/en/latest/tutorials-lca.html

This link is broken:
```
curl -L -o genbank-k31.lca.json.gz https://osf.io/zskb9/download
```

The proper link is:
```
curl -L -o genbank-k31.lca.json.gz https://osf.io/4f8n3/download
```

Currently here:
https://github.com/dib-lab/sourmash/blob/master/doc/tutorials-lca.md