[css-color] typo

--HG--
extra : rebase_source : 6e6ebdc1d6331b6dbfd462482ad49e75df15aaa8

Author: tabatkins

css-color/Overview.bs

@@ -2101,7 +2101,7 @@ Color Blending: the ''blend'' and ''blenda'' adjusters</h3>
 
 
 <h3 id='contrast-adjuster'>
-Guaranteeing Adequate Contrast: the ''constrast'' adjuster</h3>
+Guaranteeing Adequate Contrast: the ''contrast'' adjuster</h3>
 
 	Guaranteeing that foreground and background colors contrast sufficiently is important,
 	but even if one knows the mathematical definition of "appropriate contrast",