Fix Typo and Correct Object Attribute in BBS signature computation

[AlbertoSvg]

Hello,
I have reviewed the updates, and everything looks good to me, except for two minor points that this PR addresses:

1. Fixing a typo in the file draft-ietf-jose-json-proof-algorithms.md
2. Fixing the BBS signature computation:

In the bbs.fixtures.mjs file, the BBS sign function was called with an object containing a 'payloads' property. However, the sign function is designed to accept input objects of type "BbsSignRequest", which require a 'messages' property. Providing an object with a 'payloads' property instead resulted in the 'messages' property being set to 'undefined'.

No issues were immediately visible because, the BBS scheme allows empty message arrays for signing operations.
Internally, the 'sign' operation accessed the 'messages' property, which was "undefined". So basically, It successfully computed the signature over an empty array of messages.

If you test the current code as it is and pass to the function an empty array of payloads you will notice that it produces the same signature value. Additionally, if you set verifySignature: true in the deriveProof function, the signature verification would fail, as the messages are correctly passed to the function in this case.

[dwaite]

Thank you Alberto! I look forward to seeing your rust implementation!