[css-values-4] fix copypaste error. Fixes w3c#2347.

Author: tabatkins

css-values-4/Overview.bs

@@ -986,7 +986,7 @@ Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			The cap-height of a font can be found in different ways.
 			Some fonts contain reliable metrics for the cap-height.
 			If reliable font metrics are not available,
-			UAs may determine the cap-height from the height of a lowercase glyph.
+			UAs may determine the cap-height from the height of an uppercase glyph.
 			One possible heuristic is to look at
 			how far the glyph for the uppercase “O” extends below the baseline,
 			and subtract that value from the top of its bounding box.