Fix nits in comments in feature_bip68_sequence

Summary:
Just move some inline comments to their own line, and replace a BTC with
BCH.

Test Plan:

./test/functional/test_runner.py feature_bip68_sequence

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2627