Fix comment about s in schnorr sigs

Summary: s/order/n/ for consistency.

Test Plan: None

Reviewers: markblundeberg, O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2741