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

crypto/heimdal/lib/krb5/kerberos.8

@@ -31,7 +31,7 @@
 .\"
 .\" $Id$
 .\"
-.Dd September 1, 2000
+.Dd May 15, 2021
 .Dt KERBEROS 8
 .Os HEIMDAL
 .Sh NAME
@@ -71,9 +71,12 @@ or
 .Ic ftp ,
 without giving your password.
 .Pp
-For more information on how Kerberos works, and other general Kerberos
-questions see the Kerberos FAQ at
-.Pa http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html .
+For more information on how Kerberos works, see the tutorial at
+.Lk https://kerberos.org/software/tutorial.html
+or the informal
+.Dq dialogue
+at
+.Lk https://web.mit.edu/kerberos/dialogue.html .
 .Pp
 For setup instructions see the Heimdal Texinfo manual.
 .Sh SEE ALSO