[trivial] Fix recently introduced typos in comments
Summary:
backported PR9696
https://github.com/bitcoin/bitcoin/pull/9696/files
skipped some where irrelevant
Test Plan: none
Reviewers: #bitcoin_abc, jasonbcox, Fabien
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D2977