README and CONTRIBUTING.md use inconsistent legacy contribution links

[ayangabryl]

I noticed a small docs consistency issue around contribution links in this repository.

Right now the repo points readers to two slightly different legacy contribution URLs:

• README.md links to https://legacy.reactjs.org/docs/how-to-contribute.html
• CONTRIBUTING.md links to https://reactjs.org/docs/how-to-contribute.html

These may both still resolve, but the repo currently uses two different legacy domains for what appears to be the same contribution guide.

It might help to standardize these links, or clarify the canonical contribution guide location if the current setup is intentional.

Where I found it:

• https://github.com/facebook/react/blob/main/README.md
• https://github.com/facebook/react/blob/main/CONTRIBUTING.md

This is just a small docs cleanup report.

[Fashad-Ahmed]

@ayangabryl I can pick it up real quick