Update merkle.cpp

Summary:

Change comment from The reason is that if the number of hashes in the list at a given time is odd, to The reason is that if the number of hashes in the list at a given level is odd (to be a bit more precise)

This is a backport of Core PR18379

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8858