Editorial: add back syntax highlighting for observable array example

Author: domenic

index.bs

@@ -6805,8 +6805,7 @@ string "<code>ObservableArray</code>".
 <div class="example">
     The following [=IDL fragment=] defines an [=interface=] with an observable array attribute:
 
-    <!-- TODO: use highlight="idl" after parser update. -->
-    <pre>
+    <pre highlight="webidl">
         [Exposed=Window]
         interface Building {
           attribute ObservableArray&lt;Employee> employees;