[integration_test] Update outdated onScreenshot signature in README code snippet (#132409)

Fixes https://github.com/flutter/flutter/issues/126690.

Encountered while testing https://github.com/flutter/flutter/pull/132406. Updates the signature to be in line with the [current typedef](https://github.com/flutter/flutter/blob/b3096225e0ebc005ed678fa7a9f03164caba1d73/packages/integration_test/lib/common.dart#L26).

packages/integration_test/README.md

@@ -140,7 +140,7 @@ import 'package:integration_test/integration_test_driver_extended.dart';
 
 Future<void> main() async {
   await integrationDriver(
-    onScreenshot: (String screenshotName, List<int> screenshotBytes) async {
+    onScreenshot: (String screenshotName, List<int> screenshotBytes, [Map<String, Object?>? args]) async {
       final File image = File('$screenshotName.png');
       image.writeAsBytesSync(screenshotBytes);
       // Return false if the screenshot is invalid.