[css-values-4] Link fix.

Author: tabatkins

css-values-4/Overview.bs

@@ -1813,7 +1813,7 @@ Type-Checking Expressions</h5>
 	* The [=CSSNumericValue/type=] of a ''calc()'' expression
 		is the [=CSSNumericValue/type=] of its contained expression.
 		The [=CSSNumericValue/type=] of a ''min()'' or ''max()'' expression
-		is the result of [=adding two types|adding the types=]
+		is the result of [=add two types|adding the types=]
 		of its comma-separated expressions.
 		If the result is failure,
 		the entire [=math function=] is invalid.