kerberos.8: Replace dead link

Replace it with a tutorial hosted on kerberos.org and the classic
"dialogue" from Bill Bryant. The change has been reported and
merged upstream (https://github.com/heimdal/heimdal/commit/7f3445f1b7).

MFC after: 3 days
PR: 251854
Reported by: ktullavik@gmail.com
Submitted by: bjk (upstream github)
Reviewed by: bcr