docs(example): update readme with build step

[frankcalise]

Summary

I had trouble running the Example app after following the README steps. After building from the root of the monorepo, the iOS app began working so I updated the documentation steps.

Test plan

1. Clone the react-native-reanimated repository
2. Follow step exactly in Example/README.md and observe the iOS app will not start
3. Go back to the root of the monorepo and run yarn build
4. cd Example && yarn ios should now work successfully

[tomekzaw]

Thanks for the PR!