Update documentation in React Navigation example for React Native v0.64

[raychanks]

React Native@0.64 had moved the file NativeAnimatedHelper.js from react-native/Libraries/Animated/src/NativeAnimatedHelper to react-native/Libraries/Animated/NativeAnimatedHelper(without putting the file in src).

Would like to add a comment to the React Navigation example in the documentation to signal this update.

[thymikee]

Please send a PR changing it, sounds like a great opportunity for a contribution!

[raychanks]

Sure, will send a PR for it