Fixed a typo in README for better clarity and understanding (#47918)

Summary:
I was just going through the README file, and I felt that the word "repo" seems a bit informal. Thus I made the correction and made it "repository".

## Changelog:

[General] [Changed] - Fixed a typo in the README file.

Pull Request resolved: https://github.com/facebook/react-native/pull/47918

Test Plan: No specific Test Plan for these changes, as it is just a typo fix in README.md

Reviewed By: fkgozali

Differential Revision: D66414686

Pulled By: arushikesarwani94

fbshipit-source-id: 578530d2dcdf046a90bf5a5bc32ba68e8f496c81