Update stack/compose example to remove restart and links

[BretFisher]

The README example of a stack.yml file could use updating to remove legacy/confusing options:

1. Links are no longer needed as of compose v2. Since this example uses v3.1, links should be removed.
2. restart: option is only used for docker-compose, while restart_policy: is only used for stack deploy. Since it's an option setting, I'd recommend removing it from the example to avoid confusion like this question on SO.

I can submit a PR (just let me know if you agree with above and I'll submit) or someone can just update the README to read:

version: '3.1'

services:
  joomla:
    image: joomla
    ports:
      - 8080:80
    environment:
      JOOMLA_DB_HOST: joomladb
      JOOMLA_DB_PASSWORD: example

  joomladb:
    image: mysql:5.6
    environment:
      MYSQL_ROOT_PASSWORD: example

[mbabker]

Feel free to send PRs updating https://github.com/docker-library/docs/tree/master/joomla

I honestly haven't touched that repo/directory since we set this up and haven't actually had a running Docker installation for the better part of the last 2 years until very recently.

[BretFisher]

ok submitted docker-library/docs#1121