Replace segwit comment with more accurate comment

Summary: Noticed this while coding. Cleaned up the comment and made it consistent for a similar call.

Test Plan: N/A

Reviewers: deadalnix, schancel, jimpo, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1287