Incorrect instructions in celery tutorial

The ["Configuring Galaxy to use Postgres"](https://training.galaxyproject.org/training-material/topics/admin/tutorials/celeryless/tutorial.html#configuring-celery-to-use-postgres) section in the Alternative Celery Deployment for Galaxy tutorial ) is incorrect and may lead to Galaxy not starting up.

See https://github.com/galaxyproject/galaxy/issues/20809. I'll make the edits once/if we determine we are not making an exception for Celery tables (as per issue description).

Ping @hexylena (FYI)