[css3-ui] update reference to css2.1 with reference to css-backgrounds-3

Hi Tantek,

I’ve tried to be conservative when bikesheding css3-ui, and to avoid any change that would affect normative aspect of the spec, as these are best handled as separate commits.

This means I’ve kept a reference that css-ui makes to css2.1 to define border-width and border-style, even though is no longer the authoritative source on the topic. I suggest updating this with a reference to css-backgrounds-3, as it has updated definitions and is sufficiently stable.

This change can be applied with the attached patch.

 - Florian