[in_app_purchase_storekit] Updated comment (#9445)

### *Updated the comment refer to `StoreKit` instead of `Android`, in example/lib/main.dart*

### Before 
```dart
void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // When using the Android plugin directly it is mandatory to register
  // the plugin as default instance as part of initializing the app.
  InAppPurchaseStoreKitPlatform.registerPlatform();
```
### After
```dart
void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // When using the StoreKit plugin directly it is mandatory to register
  // the plugin as default instance as part of initializing the app.
  InAppPurchaseStoreKitPlatform.registerPlatform();
```

### Issue
Closes flutter/flutter#159623

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: avni-prajapati

packages/in_app_purchase/in_app_purchase_storekit/example/lib/main.dart

@@ -14,7 +14,7 @@ import 'example_payment_queue_delegate.dart';
 
 void main() {
   WidgetsFlutterBinding.ensureInitialized();
-  // When using the Android plugin directly it is mandatory to register
+  // When using the StoreKit plugin directly it is mandatory to register
   // the plugin as default instance as part of initializing the app.
   InAppPurchaseStoreKitPlatform.registerPlatform();