[css-typed-om-1] "invert" should be clear about what it means by "inverse"

[bzbarsky]

https://drafts.css-houdini.org/css-typed-om-1/#cssmath-invert step 2 substep 2 says:

and a value internal slot set to the inverse of this’s.

Presumably it means "1 divided by this's value internal slot" (multiplicative inverse). It should make that clear (and in particular it should be clear that this is not the additive inverse).