[trivial,doc] Fix memory consistency model in comment

Summary:
Updating a comment overlooked during review in #13247

Backport of Bitcoin Core PR13835
https://github.com/bitcoin/bitcoin/pull/13835

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4068