hello_world.md - $container_id variable case sensitivity

If you run the tutorial step-by-step, following error occurs:

```$ sudo docker logs $container_id
Usage: docker logs CONTAINER
Fetch the logs of a container
  -f, --follow=false: Follow log output```

This is obviously because bash variables are case-sensitive, so it mustn't be `CONTAINER_ID` above.

Docker-DCO-1.1-Signed-off-by: Mateusz Sulima <sulima.mateusz@email.com> (github: github_handle)