Editorial: Add syntax highlighting to an example. (#764)

Fixes #754.

Author: Ms2ger

index.bs

@@ -2349,9 +2349,7 @@ type=] that has a [=dictionary type=] in its [=flattened member types=].
     defines an [=interface=]
     with an operation that takes a dictionary argument:
 
-    <!-- Should be `highlight="webidl"`, but that gives bikeshed conniptions
-         because it does not know about the syntax we are introducing -->
-    <pre>
+    <pre highlight="webidl">
         dictionary LookupOptions {
           boolean caseSensitive = false;
         };