fix outdated comment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