arch.sh: work around gazebo AUR build error and correct comments
arch.sh: work around gazebo AUR build error and correct comments

PX4/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware