Make "maximum length of incoming protocol messages" comment match the code

Summary:
https://github.com/Bitcoin-ABC/bitcoin-abc/pull/42/commits/3fae5bcfcc052b3cd6451e8ffaecfec5cf333d83
Moving this documentation fix by Mustafa Al-Bassam (github user: musalbas)
from github mirror to phabricator source

Test Plan: NA

Reviewers: #bitcoin_abc, schancel, O1 Bitcoin ABC

Reviewed By: #bitcoin_abc, schancel, O1 Bitcoin ABC

Differential Revision: https://reviews.bitcoinabc.org/D846