[in_app_purchase_storekit] Updated comment

[avni-prajapati]

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

Before

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

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 https://github.com/flutter/flutter/issues/159623

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