update README about ndkVersion.

The error can look like this on older versions:

Your project is configured with Android NDK 26.3.11579264, but the following plugin(s) depend on a different Android NDK version:
- onesignal_flutter requires Android NDK 27.0.12077973
Fix this issue by using the highest Android NDK version (they are backward compatible).
Add the following to /Users/nanli/Documents/GitHub/OneSignal-Flutter-SDK/example/android/app/build.gradle.kts:

    android {
        ndkVersion = "27.0.12077973"
        ...
    }

Author: nan-li

example/README.md

@@ -24,6 +24,8 @@ flutter emulators --launch Medium_Phone_API_35;
 
 And then you can run `flutter run`.
 
+If you are using an older Flutter or Android Studio version, you may see an error about `ndkVersion`. Follow the instructions to update the version in `build.gradle.kts`.
+
 ## Getting Started
 
 For help getting started with Flutter, view our online