doc: use initdbArgs in example postgresql upgrade script

it's likely that the configured initdbArgs will still be relevant after the upgrade.

nixos/modules/services/databases/postgresql.md

@@ -196,6 +196,7 @@ For an upgrade, a script like this can be used to simplify the process:
       newPostgres = pkgs.postgresql_13.withPackages (pp: [
         # pp.plv8
       ]);
+      cfg = config.services.postgresql;
     in pkgs.writeScriptBin "upgrade-pg-cluster" ''
       set -eux
       # XXX it's perhaps advisable to stop all services that depend on postgresql
@@ -205,12 +206,12 @@ For an upgrade, a script like this can be used to simplify the process:
 
       export NEWBIN="${newPostgres}/bin"
 
-      export OLDDATA="${config.services.postgresql.dataDir}"
-      export OLDBIN="${config.services.postgresql.package}/bin"
+      export OLDDATA="${cfg.dataDir}"
+      export OLDBIN="${cfg.package}/bin"
 
       install -d -m 0700 -o postgres -g postgres "$NEWDATA"
       cd "$NEWDATA"
-      sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"
+      sudo -u postgres $NEWBIN/initdb -D "$NEWDATA" ${builtins.concatStringsSep " " cfg.initdbArgs}
 
       sudo -u postgres $NEWBIN/pg_upgrade \
         --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \