Fix typo in ecmult_const_impl.h

Summary:
Fix small typo in the reference given for the wNAF method

This is a backport fo libsecp256k1 PR742

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7585