[css-highlight-api] fix paired defaults terminology (w3c#7842)

Author: delan

css-highlight-api-1/Overview.bs

@@ -320,7 +320,7 @@ Applicable Properties</h4>
 Default Styles</h4>
 
 	UAs must not define any default UA stylesheet rules
-	or <a spec=css-pseudo>default highlight colors</a>
+	or <a spec=css-pseudo>paired default highlight colors</a>
 	for any [=custom highlight pseudo-elements=].
 	In other words,
 	when some content is highlighted by an unstyled custom highlight,