[css-values-4] Fix error in addition fallback definition. web-platform-tests/wpt#19160

Author: fantasai

css-values-4/Overview.bs

@@ -439,7 +439,7 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 	If a value type does not define a specific procedure for <a>addition</a>
 	or is defined as <dfn export>not additive</dfn>,
 	its <a>addition</a> operation is simply
-	<var>V<sub>result</sub></var> = <var>V<sub>a</sub></var>.
+	<var>V<sub>result</sub></var> = <var>V<sub>B</sub></var>.
 
 	If a value types does not define a specific procedure for <a>accumulation</a>,
 	its <a>accumulation</a> operation is identical to <a>addition</a>.