Updated Shaft for "Ender-3 Bearings Filement Guide"

Partial remix from https://www.thingiverse.com/thing:3035501

The original shaft was not tight enough.

I printed the 1680 version using a 688ZZ bearing.
The shaft was a bit undersized so I made my own version with tight specs to ensure the bearing is turning and not the shaft. Also designed a slightly different shape. The shaft is now exactly 8mm and should be a snug fit.