specs: update Lexicon reference "fragment" syntax to allow trailing digits

[bnewbold]

Current specs are quite restrictive: https://atproto.com/specs/nsid#:~:text=The%20fragment%20identifier%20string%20(after%20the%20%23)%20has%20the%20same%20syntax%20restrictions%20as%20the%20final%20segment%20of%20an%20NSID%3A%20ASCII%20alphabetic

We definitely have at least "v2" use-cases in the wild.

I think what needs to change here is the spec, not the records. We want these to be valid programming symbols (variable/method names), but trailing digits should be fine (as long as there are no dashes, underscores, or leading digits).

Reported here: https://bsky.app/profile/nekorug.moe/post/3lfnkukmxlc2o