[editorial] Adjust link into in-toto attestation spec

[chalin]

As I work on

• [CI] Check links with references into GitHub opentelemetry.io#5951

I hit a 404 for the following link (the link is valid, but not in the way that it gets processed by the website scripts, but that's secondary -- bare with me :):

          The full [hash value (see glossary)](https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-5.pdf),
          of the built attestation. Some envelopes in the software attestation
          space also refer to this as the [digest](https://github.com/in-toto/attestation/blob/main/spec/README.md#in-toto-attestation-framework-spec).

Regardless, the link for "digest" is into the spec in general. The spec has no entry for "digest", hence I suggest just linking to the spec and mentioning "digest" as a term. See the proposed change in the PR for the full details.

/cc @svrnm @tiffany76 @theletterf