modify arch-update header to help displaying battery-upower help
modify arch-update header to help displaying battery-upower help

Modify the header of arch-update.py so that when using `bumblebee-status
-l modules`, the content of arch-update and batter-upower will be
separated (i.e., one more blank line between the doc of arch-update and
battery-upower).

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status