Corner Cap for Prusa i3

Remixed cap from [Alternative prusa i3 corner by odino](http://www.thingiverse.com/thing:904803/#remixes):
* Lower height
* Bolt head chamfer
* Bigger hole for M3 bolt;

My Y carriage was hitting the cap, so I modified it to be lower. This is 6mm tall, the original is 9mm (plus bolt head height).