MC-4126: Duplicate row border after switching Row appearance

- Fix TS error with observable element items

Author: davemacaulay

app/code/Magento/PageBuilder/view/adminhtml/web/ts/js/content-type/observable-object.d.ts

@@ -4,5 +4,7 @@
  */
 
 export default interface ObservableObject {
-    [key: string]: KnockoutObservable<{}>;
+    [key: string]: {
+        [key: string]: KnockoutObservable<any>;
+    };
 }