[css-conditional] Make the 'box-shadow' example make more sense, since I was thinking more of 'text-shadow' when I wrote it.

Fixes http://lists.w3.org/Archives/Public/www-style/2013Mar/0037.html
[css3-conditional] Also use better class name in example (followup to previous commit).

Continues fixing http://lists.w3.org/Archives/Public/www-style/2013Mar/0037.html