[css-values-4][editorial] fix markup

Author: tabatkins

css-values-4/Overview.bs

@@ -578,7 +578,7 @@ Representing Interpolated Values: the ''mix()'' notation</h3>
 	rather than the more typical comma
 	because the values themselves can contain commas.
 
-	If the two <<declaration-value>s in ''mix()'' are [=interpolation|interpolable=]
+	If the two <<declaration-value>>s in ''mix()'' are [=interpolation|interpolable=]
 	(without using ''mix()'' itself)
 	as values for the property in which it is specified,
 	the [=computed value=] of ''mix()'' is