Fix BlockAssembler::AddToBlock logging

Summary:
This is a backport of core#23050

The other erroneous message fixed in the source material was never wrong in Bitcoin ABC (see core#9602 and D4812)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12168