Fix typos in libsecp256k1

Summary: Backport of PR#440 by practicalswift

Test Plan:

make check

Reviewers: schancel, MattBitprim, #bitcoin_abc, Mengerian

Reviewed By: schancel, #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D780