docs: fix --healthcheck-command option

[stefanb2]

Make the documentation agree with the code.

Related #3507

[openshift-ci-robot]

Hi @stefanb2. Thanks for your PR.

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[rh-atomic-bot]

Can one of the admins verify this patch?
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[mheon]

/ok-to-test
/approve

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mheon, stefanb2

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[mheon]

Commit needs a signoff - please run git commit --amend -s and re-push

[rhatdan]

/lgtm
Thanks @stefanb2