Need remind error when pass illegal completion subcommand

[chunfuwen]

1. execute command: sudo podman run --rm -it --privileged --entrypoint=/usr/bin/bootc-image-builder quay.io/centos-bootc/bootc-image-builder:latest completion test
2. it need throw error since bib doesn't support completion test subcommand
3. But actually it normally output as below

root@xx bootc-image-builder]# sudo podman run --rm -it --privileged --entrypoint=/usr/bin/bootc-image-builder quay.io/centos-bootc/bootc-image-builder:latest completion test
Generate the autocompletion script for bootc-image-builder for the specified shell.
See each sub-command's help for details on how to use the generated script.

Usage:
  bootc-image-builder completion [command]

Available Commands:
  bash        Generate the autocompletion script for bash
  fish        Generate the autocompletion script for fish
  powershell  Generate the autocompletion script for powershell
  zsh         Generate the autocompletion script for zsh

Flags:
  -h, --help   help for completion

Global Flags:
      --log-level string   logging level (debug, info, error); default error

Use "bootc-image-builder completion [command] --help" for more information about a command.