Update import style shown in README

[chriszs]

Description

One Line Summary

Updates the import style shown in the README to match the current behavior.

Details

Motivation

The wrong import could confuse users who try to implement the SDK.

Testing

Manual testing

I installed react-native-onesignal 5.0 and onesignal-expo-plugin 2.0 and found that importing OneSignal direct from the library as I did before gave an undefined error. After consulting more documentation, I changed it to this style and it worked.

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• [x] Any Public API changes are explained in the PR details and conform to existing APIs

[chriszs]

I now see that this is a dupe of #204. Only difference is whitespace.