Fix incorrect pg_autoctl set command in tutorial

[thara]

In Editing the replication settings while in production section, describing how to set a node’s candidate priority, the command example is missing the node subcommand.

The current example causes an error:

$ docker compose exec node3 pg_autoctl set candidate-priority 0 --name node3
pg_autoctl set: candidate-priority: unknown command

Available commands:
  pg_autoctl set
  + node       set a node property on the monitor
  + formation  set a formation property on the monitor

This PR updates the tutorial to use the correct command syntax.