[css-nav-1] Modify the IDL format

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -602,9 +602,10 @@ To create an instance of the {{NavigationEvent}} interface, use the {{Navigation
 passing an optional {{NavigationEventInit}} dictionary.
 
 <pre class=idl>
-[Exposed=Window,
-  Constructor (DOMString type, optional NavigationEventInit eventInitDict)]
+[Exposed=Window]
 interface NavigationEvent : UIEvent {
+    constructor(DOMString type,
+                optional NavigationEventInit eventInitDict);
     readonly attribute SpatialNavigationDirection dir;
     readonly attribute EventTarget? relatedTarget;
 };