Update expo instructions

[kadikraman]

What, How & Why?

1. use npx expo install instead of npm install

npx expo install will use your configured package manager, npm install will use npm so won't work if you're using a different package manager

2. update expo go instructions

The instructions said "consider using development build" which implies there are other ways to get it to work within Expo Go, which isn't the case. This adds a small tweak to make it clearer what to do next.

[cla-bot]

Realm welcomes all contributions! The only requirement we have is that, like many other projects, we need to have a Contributor License Agreement (CLA) in place before we can accept any external code. Our own CLA is a modified version of the Apache Software Foundation’s CLA. Our records show that CLA has not been signed by @kadikraman. Please submit your CLA electronically using our Google form so we can accept your submissions. After signing the CLA you can recheck this PR with a @cla-bot check comment. The GitHub usernames you file there will need to match that of your Pull Requests. If you have any questions or cannot file the CLA electronically, make a comment here and we will be happy to help you out.

[kadikraman]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[kraenhansen]

Thanks a lot @kadikraman!