[css-highlight-api]: Remove redundant add method from HighlightRegistry. (#6092)

Author: sanketj

css-highlight-api-1/Overview.bs

@@ -226,13 +226,12 @@ Registering Custom Highlights</h3>
 	[Exposed=Window]
 	interface HighlightRegistry {
 		setlike<Highlight>;
-		HighlightRegistry add(Highlight value);
 	};
 	</xmp>
 
 	<div algorithm="to register a custom highlight">
 		To [=register=] a [=custom highlight=],
-		invoke the {{HighlightRegistry/add()}} of the [=highlight registry=]
+		invoke the {{HighlightRegistry/add()}} method of the [=highlight registry=]
 		with the [=custom highlight=] as the argument.
 
 		When invoked,