[css-values-4] Switch heading ID to concept

Author: fantasai

css-values-4/Overview.bs

@@ -7,7 +7,7 @@ Status: ED
 Work Status: Revising
 ED: https://drafts.csswg.org/css-values-4/
 TR: https://www.w3.org/TR/css-values-4/
-Previous Version: https://www.w3.org/TR/2021/WD-css-values-4-20210715/
+Previous Version: https://www.w3.org/TR/2021/WD-css-values-4-20210930/
 Previous Version: https://www.w3.org/TR/2021/WD-css-values-4-20210715/
 Previous Version: https://www.w3.org/TR/2020/WD-css-values-4-20201111/
 Previous Version: https://www.w3.org/TR/2019/WD-css-values-4-20190131/
@@ -465,7 +465,7 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 	If a value type does not define a specific procedure for <a>accumulation</a>,
 	its <a>accumulation</a> operation is identical to <a>addition</a>.
 
-<h3 id="mix">
+<h3 id="interpolate">
 Representing Interpolated Values: the ''mix()'' notation</h3>
 
 	[=Interpolation=] of two values can be represented