docs(examples): update React Native example

This includes `engine.io-client@4.0.3`, which fixes two issues with
React Native.

See also:

- https://github.com/socketio/engine.io-client/commit/177b95fe463ad049b35170f042a771380fdaedee
- https://github.com/socketio/engine.io-client/commit/ccb99e3718e8ee2c50960430d2bd6c12a3dcb0dc

[skip ci]