Fixed command plural

What does this MR do?

On 13.4.2 the command is the --skip-preflight-checks option is plural, not singular as shown in the docs:

gitlab-ctl promote-to-primary-node --help
Usage: gitlab-ctl promote-to-primary-node [options]
    -p                               Do not ask for confirmation that primary is down
        --[no-]confirm-primary-is-down
    -m, --skip-preflight-checks      Perform promotion without running any preflight checks
    -f, --force                      Proceed even if preflight checks fail

Related issues

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