Updated Prusa i3 Mk2/Mk2S/Mk2.5 Y distance

This is an updated version of the Y motor spacer (Prusa calls it "y distance"). Original Mk2 came out without this part, and the stress on the Y motor holder could bend or even crack the plastic part. In the Mk2S kit, Prusa added "y distance" that is supposed to solve this problem. I was building another Mk2S and found that part to be too thin. The Y motor should be parallel to the threaded rods when it rests on the spacer, and it wasn't, so the tension of the belt was still bending the Y motor