Print usage for 'help' command.

[michaelgrahamevans]

The terminusdb-container script mentions a help command in it's usage string but running it prints invalid command (as well as the usage string). This change cleans the output up slightly by expecting a help command input.

./terminusdb-container help

invalid command

USAGE:
  terminusdb-container [COMMAND]

  help        show usage
  run         run container
  stop        stop container
  console     launch console in web browser
  attach      attach to prolog shell
  exec        execeute a command inside the container
  rm          remove volumes

[rrooij]

Thanks a lot! Seems like we missed this one indeed.