Update persistent-screen.md with the cern-get-keytab command

[rkansal47]

Updating the instructions for creating a kerberos keytab for persistent screen or tmux authentication using the newer cern-get-keytab instead of the keytab utility, since the latter did not work for me.

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[bendylan]

Ha, snap! I also raised a PR for this, though I think the ktutil option should not be included as-is, as it's suggesting the use of a deprecated enctype (arcfour-hmac-md5)

[rkansal47]

Oh oops, I didn't see that! Closing this one then.