[llvm] [HashRecognize] Clarify hdr comment on GF(2^n) (NFC) (PR #157482)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> How is `n` related to `m` ? I'm confused about `2^m` in the commit message while it doesn't occur in the comment.

Sorry, fixed thinko: m is the order of the generating polynomial, and n is the order of the Galois field, per our conventions.

https://github.com/llvm/llvm-project/pull/157482