Improve plist_caveats and brew-services

1. Use `brew services` to replace `launchctl` in plist_caveats.
2. Minor bug fix for brew-services.

Note there's still a bug in brew-services, see https://github.com/Homebrew/homebrew/issues/30628.
I think brew-services is better as it's simple than current instructions and more friendly for uninstall and clean .plist files.