[css-properties-values-api] Editorial link fix.

css-properties-values-api/Overview.bs

@@ -371,7 +371,7 @@ rather than the original token sequence used to produce that value.
 Any ''var()'' function that references a registered custom property
 must be replaced with an <dfn export>equivalent token sequence</dfn>,
 which is equal to the token sequence that would have been produced
-by [[cssom#serialize-a-css-value|serializing]] the computed value,
+by [=serialize a CSS value|serializing=] the computed value,
 and [[css-syntax-3#tokenization|tokenizing]] the resulting string.
 
 <div class='example'>