Added correct example app name

[thePianoKid]

Summary

CONTRIBUTING.md had multiple references to this repo's example project. In CONTRIBUTING.md, the example project was supposed to belong in the /example directory, when it was actually in the /example-app directory. The /example directory does not exist.

Motivation

This is a tiny correction, but it adds a bit of unnecessary friction to the onboarding process :)

Testing

N/A - all changes were made to docs. None of the functionality was changed.

• I tested this manually
• I added automated tests

Documentation

Select one:

• I have added relevant documentation for my changes.
• This PR does not result in any developer-facing changes.

[CLAassistant]

All committers have signed the CLA.

[thePianoKid]

👇 Bitrise is failing, but my change doesn't effect the functionality... am I doing something wrong?

[nazli-stripe]

@thePianoKid thanks for the PR! Bitrise requires admin approval to run the tests, which I just approved. It should pass as this is just a ReadMe change, after which I'll merge the PR!