Document correct registry port to test SSL

What does this MR do?

In https://gitlab.com/gitlab-com/support/support-pairing/-/issues/990 @nelbacha and me discovered a likely incorrect openssl command in our docu. Shouldn't the port shown for Omnibus-based installs be the same as for shown source installs?

Related issues

As this was also updated for 13.1, should it be ported to 13-0-stable-ee as well?

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