Wrong data for preferences in container/service deploy

[thejoelinux]

File: compose/compose-file/deploy.md

I've tried to put this:

    deploy:
      placement:
        preferences:
          - datacenter=us-east

in my docker-compose.yml file, it doesn't work and raise the error : services.my-service.deploy.placement.preferences.0 must be a mapping

After some research, it turns out this is not the way preferences work.