[css-color] incorrect contrast ratio calculation

Section 10.5 includes an incorrect formula for calculating contrast ratio:

(L1 + 0.05) / (L2 + 0.05)

where L1 is the smaller of the two luminances, and L2 is the larger.

According to http://www.w3.org/TR/WCAG20/#contrast-ratiodef
L1 is the *lighter* of the two, and lighter colors have a *larger* relative
luminance (1 white, 0 black)

so it should read:

where L1 is the larger of the two luminances, and L2 is the smaller.