Update the center dock to add adjustability

[datapaganism]

Hi Zruncho,

I love the ZeroClick but I was having issues with the center dock, My hotend would not line up by a few milimeters in the Y dimension, I decided to modify the dock a little bit.

I have changed the holes that go to the 1515 with slots that should add about 3mm of adjustability.
I fixed the chamfer in one of the screw holes because it was wrong originally and you needed mismatching bolts.

I also trimmed away 3.5mm from one of the edges to clear the linear rail carridge if you use the slots like I have, I did my best to match the design asthetics of the original with the chamfers and fillets but it might need some tweaking.

With these changes it has improved the attaching in my use case, I though maybe others would benefit from it too

Let me know what you think

Thanks