Fix subcommand groups in top level help.

[jglogan]

• Closes [Bug]: UX - CLI top-level subcommands scrambled at the end #349.

[jglogan]

@egernst Tested fix with your plugin:

$ container --help
OVERVIEW: A container platform for macOS
USAGE: container [--debug] <subcommand>
OPTIONS:
  --debug                 Enable debug output [environment: CONTAINER_DEBUG]
  --version               Show the version.
  -h, --help              Show help information.
CONTAINER SUBCOMMANDS:
  create                  Create a new container
  delete, rm              Delete one or more containers
  exec                    Run a new command in a running container
  inspect                 Display information about one or more containers
  kill                    Kill one or more running containers
  list, ls                List containers
  logs                    Fetch container stdio or boot logs
  run                     Run a container
  start                   Start a container
  stop                    Stop one or more running containers
IMAGE SUBCOMMANDS:
  build                   Build an image from a Dockerfile
  images, image, i        Manage images
  registry, r             Manage registry configurations
OTHER SUBCOMMANDS:
  builder                 Manage an image builder instance
  network, n              Manage container networks
  system, s               Manage system components
PLUGINS:
  my-plugin               Super rad plugin (Eric)