[css-extensions] Fix link error.

Author: tabatkins

css-extensions/Overview.bs

@@ -49,7 +49,7 @@ Extension Names</h2>
 	A <dfn>declarative custom selector</dfn> is defined with the ''@custom-selector'' rule:
 
 	<pre class='prod'>
-	<dfn>@custom-selector</dfn> = @custom-selector <<custom-selector>> <<selector>> ;
+	<dfn>@custom-selector</dfn> = @custom-selector <<custom-selector>> <<selector-list>> ;
 	<dfn>&lt;custom-selector></dfn> = <<custom-arg>>? : <<extension-name>> [ ( <<custom-arg>>+#? ) ]? ;
 	<dfn>&lt;custom-arg></dfn> = $ <<ident-token>> ;
 	</pre>
@@ -84,7 +84,7 @@ Extension Names</h2>
 	</div>
 
 	This defines a <a>custom selector</a> which is written as a <a spec=selectors>pseudo-class</a> with the given <<extension-name>>,
-	and represents a '':matches()'' selector using the provided <<selector>> as its argument.
+	and represents a '':matches()'' selector using the provided <<selector-list>> as its argument.
 
 	<div class='example'>
 		For example, if an author wanted to easily refer to all heading elements in their HTML document,