Thread example doesn't work with diferent radius

[mantielero]

https://github.com/CadQuery/cadquery-contrib/blob/3902f2be08fe9bbbf70773a198730fb07e2b3d39/examples/Thread.py#L58

If you modify the radius, the core won't be affected.

It should be something like:

core = cq.Workplane("XY",origin=(0,0,-d)).circle(radius).circle(radius-1).extrude(height+1.75*d)