docs: update expo guide

[vonovak]

Description

I wanted to mention npx expo prebuild in the docs because it's an important step in making sure that the config plugins are applied.

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

---

Think react-native-firebase is great? Please consider supporting the project with any of the below:

• 👉 Star this repo on GitHub ⭐️
• 👉 Follow React Native Firebase and Invertase on Twitter

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
react-native-firebase	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 20, 2024 6:43am