Correct the keystone cutout dimensions

[jbmorley]

Right now the dimensions of the keystone cutouts are wrong; the fascia cutouts almost certainly only work because of the laser kerfing which I'd not accounted for when cutting the fascia (and would like to be able to correctly account for in the future), and the mounting plate cutouts are too narrow making it very hard to clip the keystones in place. I've added a diagram showing one manufacturer's expected tolerance for this plate to the readme (https://github.com/jbmorley/little-luggable#useful-references) and it seems like all dimensions are a little small; it'd probably be best to pick something in the middle of the range.

Once this has been done, both the screen fascia and mounting plate should be re-exported correctly adjusted for laser and aluminium kerfing.