Remove useless comma (#4711)

Found an additional comma typo when I implemented this example with typescript.

Author: mass2527

beta/src/pages/learn/choosing-the-state-structure.md

@@ -1050,7 +1050,7 @@ export const initialTravelPlan = {
   35: {
     id: 35,
     title: 'Oceania',
-    childIds: [36, 37, 38, 39, 40, 41,, 42],   
+    childIds: [36, 37, 38, 39, 40, 41, 42],   
   },
   36: {
     id: 36,
@@ -2849,4 +2849,4 @@ Keep in mind that you [should not mutate objects in state](/learn/updating-objec
 
 </Solution>
 
-</Challenges>
+</Challenges>