fix docs to use cern-get-keytab

[bendylan]

The existing docs with ktutil use the wrong enctypes and will fail. However rather than fixing that, I think using cern-get-keytab is probably the right thing to suggest, since that tool takes care of the details.

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[jonas-eschle]

Hey, thanks for contributing and sorry for the long silence! I am unfortunately rather unfamiliar with the section, are you confident that the change is the right thing? If so, lgtm

[goi42]

I just opened #66 without realizing this PR already existed--sorry about that. I'll leave it open for now since it makes some additional changes not included here.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.

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