Improved build plate adapter for Nii autochanger

Remix of Nii's automatic plate changer, found here:

https://makerworld.com/models/925870-auto-build-plate-changer-for-a1-mini#profileId-901254

This is a modified version of the build plate adapter, specifically. 

I slightly shortened the rear side of the adapter and added a 9° taper and 1mm chamfer to help the plate fully seat onto the bed without getting caught on the edge, which was a problem I had with the original version. 

I also added two small protrusions on the bottom to help the plate slide across the bed, since the M3x6 screws were scratching the front edge of the magnetic bed layer for me. 

Enjoy!