[values-4] Fix typo: plural mistake

Author: arayaryoma

css-values-4/Overview.bs

@@ -443,7 +443,7 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 	its <a>addition</a> operation is simply
 	<var>V<sub>result</sub></var> = <var>V<sub>a</sub></var>.
 
-	If a value types does not define a specific procedure for <a>accumulation</a>,
+	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>.
 
 <h2 id="textual-values">