Clarify how createVerifyData calls RDF canonicalization

[jyasskin]

https://www.w3.org/TR/vc-di-bbs/#createverifydata says

Initialize proofHash to the result of performing RDF Dataset Canonicalization [RDF-CANON] on the proof options, i.e., the proof portion of the document with the proofValue removed.

I assume "the document" is the input declared as "The inputs include a JSON-LD document (document)". "The proof options" doesn't seem to have a defined meaning except in the "i.e." clause, so you can probably replace it with that clause. "the proof portion of the document" doesn't seem to have a defined meaning, and proofValue is also not defined.