[css-scrollbars] Rephrase stale statement

The property used to have multiple keyword values, in addition to
explicit colors. Now it only has one. Text that refers to "the keyword
values" had therefore become strange, and it is better to be explicit
about which keyword value we are talking about: auto.

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -273,7 +273,7 @@ Value Definitions</h3>
 
 	When using 'scrollbar-color' property with specific color values,
 	authors should ensure the specified colors have enough contrast between them.
-	For keyword values, UAs should ensure the colors they use have enough contrast.
+	For the ''scrollbar-color/auto'' value, UAs should ensure the colors they use have enough contrast.
 	See
 	<a href="https://www.w3.org/TR/WCAG21/#non-text-contrast">WCAG 2.1 SC 1.4.11 Non-text Contrast</a>
 	[[WCAG21]].