Fix RNTester Path (#29786)

Summary:
* Corrects running on android instructions for rn-tester
 * Corrects path for rn-tester in package.json, contributing.md and .buckconfig

## Changelog

[General] [Fixed] - Fix rn-tester path in documentation and configs

Pull Request resolved: https://github.com/facebook/react-native/pull/29786

Reviewed By: hramos

Differential Revision: D23441274

Pulled By: rickhanlonii

fbshipit-source-id: 9454679335d3794b59a1e4c2e7eae23ed348a427