Screw location for Pi cam can result in short

Due to screw head size and placement, using top screw holes on Pi cam can cause a short. Using bottom two holes avoids this potential issue.

Reported by @Ezward on the Donkey Car Slack: https://donkeycar.slack.com/archives/C487786DC/p1522638679000068?thread_ts=1522442985.000360&cid=C487786DC