bash-completion: Don’t misidentify short options as commands to `apt`

Hello there,

I hope it’s fine to do this as a direct merge request without a corresponding bug report. First time contributing anything here/to Debian :-)

This fixes a tiny inconvenience in the bash-completion for apt. If one started a command line with e.g. apt -s, no further options or commands would get suggested afterwards. (Found this while answering in the German Debianforum.)