tweak psql procedure so it actually works

The $() was being interpolated by the caller, and the
pg_upgradecluster command was incomplete.

howto/upgrades/buster.md

@@ -120,7 +120,7 @@ PostgreSQL is special and needs to be upgraded manually.
 
  1. make a full backup of the old cluster:
 
-        ssh bungei.torproject.org "sudo -u torbackup postgres-make-one-base-backup $(grep ^meronense.torproject.org $(which postgres-make-base-backups ))"
+        ssh -tt bungei.torproject.org 'sudo -u torbackup postgres-make-one-base-backup $(grep ^meronense.torproject.org $(which postgres-make-base-backups ))'
 
     The above assumes the host to backup is `meronense` and the backup
     server is `bungei`. See [howto/postgresql](howto/postgresql) for details of that
@@ -137,7 +137,7 @@ PostgreSQL is special and needs to be upgraded manually.
         printf "about to drop cluster main on postgresql-11, press enter to continue" &&
         read _ &&
         pg_dropcluster --stop 11 main &&
-        pg_upgradecluster -m upgrade -k &&
+        pg_upgradecluster -m upgrade -k 9.6 main &&
         for cluster in `ls /etc/postgresql/9.6/`; do
             mv /etc/postgresql/9.6/$cluster/conf.d/* /etc/postgresql/11/$cluster/conf.d/
         done