Fix selfsign.image.* values documentation

What does this MR do?

The documentation for selfsign.image.* was incorrect and did not work. This documents it to match the usage here: https://gitlab.com/gitlab-org/charts/gitlab/-/blob/a53917db75b622d02a2d216c4e33da1ebb7f1c1f/charts/shared-secrets/templates/_self-signed-cert-job.yml#L30-33

Related issues

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Review checklist

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1. Primary Reviewer

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