Sync usage with reality

cassh.sh

@@ -17,7 +17,8 @@ usage()
 Usage:
 	${0##*/} issue [-hqv] [-I key_id] [-n principals]
 	    [-V validity_interval]
-	${0##*/} mkfile [-n principals] authorized_keys | known_hosts
+	${0##*/} mkfile authorized_keys [options]
+	${0##*/} mkfile known_hosts [hostnames]
 EOF
 	exit 1
 }