Update three.d.ts

http://threejs.org/docs/#Reference/Extras.Helpers/BoundingBoxHelper
.box property type is THREE.Box3, not the array. Seems mistake.

threejs/three.d.ts

@@ -5613,7 +5613,7 @@ declare module THREE {
         constructor(object: Object3D, hex?: number);
 
         object: Object3D;
-        box: Box3[];
+        box: Box3;
 
         update(): void;
     }