Stale docs URL in internet_identity.did comment #3888

Summary

Line 518 of src/internet_identity/internet_identity.did references:

```
// See https://internetcomputer.org/docs/current/references/ic-interface-spec/#signatures for
```

internetcomputer.org/docs is the retired portal site. The IC interface spec now lives at:

https://docs.internetcomputer.org/references/ic-interface-spec/#signatures

Impact

We sync ii-spec.mdx and internet_identity.did into the ICP developer docs via a script. The stale URL inside the Candid code block gets inlined verbatim into the rendered page, pointing readers to the retired site.

Suggested fix

// See https://docs.internetcomputer.org/references/ic-interface-spec/#signatures for

Happy to open a PR if that helps.