i3 rework z-limit with M2.5 nuts

Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3628141.

Wanted to add a 2nd limit switch to use independendt drives for each Z motor, and I wanted the screw head on the outside of the switch so the switch can be removed easily, for this I needed M2.5 screws instead of M3 that thread into the switch body.