Update README.md with refined grammar and formatting in contributing guidelines

[ismailbentabett]

Description

This pull request refines the grammar and formatting in the contributing guidelines for the reactjs.org repository. The changes focus on improving clarity and consistency, making it easier for contributors to follow the instructions.

Changes Made

• Clarified language and instructions in the "Getting Started" and "Contributing" sections.
• Adjusted wording for better readability in various steps and guidelines.
• Ensured consistency in terminology and formatting throughout the document.

Checklist

• [ x] I have tested the changes locally to ensure accuracy.
• [ x] I followed the contributing guidelines.
• [ x] The code is properly formatted and adheres to the existing style.
• [ x] All tests pass.
• [ x] I have included relevant documentation for the changes made.

Please review and merge. Thank you!

[AhmedBaset]

I think this should be landed to the upstream reactjs/react.dev