[css-transforms-2] Variable `dot` not defined in interpolation of decomposed 3d matrix

[smfr]

From https://www.w3.org/Bugs/Public/show_bug.cgi?id=29815:

In https://drafts.csswg.org/css-transforms/#interpolation-of-decomposed-3d-matrix-values, it says theta = acos(dot). However, dot is never defined here.

It probably should be product

h/t https://github.com/canaltinova for finding this.

[dbaron]

I checked that this agrees with the relevant Chromium code. That checking does require use of the trigonometric identity for sin(⍺-β).

I'll fix this, and simplify away a "times 1" while I'm there.