Make Z Coupler Pin spacing consistent

[Poofjunior]

sunkcost on Discord has pointed out that the back Z coupler spacing is different from the front two.

While this doesn't affect the machine's functionality, it is confusing from a "what-was-intended" or "does-this-matter" point of view. It also turns out that one of these two different spacings is easier to 3D print (out of PETG).

With that said, let's make the dowel pin spacing consistent at 12mm for each coupler. Doing so will cause less confusion and make the part more printable from PETG. Although PETG isn't a vetted material to print all parts from, this is one step closer to getting it to that point.

[Poofjunior]

Mission accomplished!