docs: update scripts in contributing.md

[jblarriviere]

Summary

The commands for testing the example app, given in contributing guidelines, were outdated

Test Plan

What's required for testing (prerequisites)?

NA

What are the steps to reproduce (after prerequisites)?

NA

Compatibility

OS	Implemented
iOS	✅❌
Android	✅❌

Checklist

• [X] I have tested this on a device and a simulator
• [X] I added the documentation in README.md
• [X] I updated the typed files (TS and Flow)

[vonovak]

thanks for the PR! 👍