Merge #10258: Fixed typo in documentation for merkleblock.h

Summary:
dd07068 Fixed typo in documentation for merkleblock.h (Mikerah)

Tree-SHA512: 92655c8022eb33852c116da517b12bd17b3b668713cd85fe185a15245ea9810605626b1d0955fa117f7b56863e0a52b3a86dab42409332452b5eb72a7d34f30d

Backport of Core PR10258
https://github.com/bitcoin/bitcoin/pull/10258/files

Test Plan: Read it.

Reviewers: Fabien, Mengerian, markblundeberg, #bitcoin_abc, deadalnix

Reviewed By: markblundeberg, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3008