fix outdated comment
Summary: The data structure has been renamed and refactored at least twice since this comment was written. mapMultiArgs no longer exists.
Test Plan: grep -r mapMultiArgs . now shows only one match in some old release note.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13016