docs: Fix dead links in README for rn-tester (#31901)

Summary:
Part of https://github.com/facebook/react-native/issues/31788

~Updated link in README that was pointing to master branch to main branch~

Realized that link in rn-tester README and ReactAndroid README leads to a dead link, so I've fixed the links

## Changelog

<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->

[INTERNAL] [FIXED] - Fix dead links in README

Pull Request resolved: https://github.com/facebook/react-native/pull/31901

Test Plan: - [ ] Updated link directs you to appropriate page

Reviewed By: PeteTheHeat

Differential Revision: D29933044

Pulled By: GijsWeterings

fbshipit-source-id: c1f301626acbb2995d74f78d8bc19214c70e9319