Merge #12797: init: Fix help message for checkblockindex

Summary:
4ae7d15 init: Fix help message for checkblockindex (MarcoFalke)

Pull request description:

Minor fixup for my commit fa6ab96799f9d7946200fb646fefe35c6daab9b2.

Tree-SHA512: 18f9255bf1342007be2bdc26d6f688bcd27ba8eebfc709bd9ee31dfd2e4d955d2b699686492ccf59e94eb4b1cc7bf3332376aa151a68cb0b21695b3f67d4a940

Backport of Core PR12797
https://github.com/bitcoin/bitcoin/pull/12797/

Test Plan:

make check
./bitcoind --help -help-debug

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3813