typo in comment for secp256k1_ec_pubkey_tweak_mul ()

Summary:
Fixes a typo in secp256k1.h documentation

This is a backport of linsecp256k1's PR631

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5348