Update Overview.bs (#13466)

types attribute is marked as [SameObject] but not readonly, which is invalid

Author: gb-jos

css-view-transitions-2/Overview.bs

@@ -692,7 +692,7 @@ the active view transition of an element is exposed to script via a property on
 			readonly attribute Promise<undefined> ready;
 			readonly attribute Promise<undefined> finished;
 			undefined skipTransition();
-			[SameObject] attribute ViewTransitionTypeSet types;
+			[SameObject] readonly attribute ViewTransitionTypeSet types;
 			readonly attribute Element transitionRoot;
 			undefined waitUntil(Promise<any> promise);
 		};