Update README.md to fix citation link

[samGreer]

PR summary

Closes #27782. The link to the citation page on the read me file was broken, so I reverted it back to the previous link (https://matplotlib.org/stable/users/project/citing.html) which seems to work.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

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[rcomer]

Given that two different people have raised the problem, I think it is worth doing this. It will be trivially easy to switch back once v3.9 is released. I will leave it unmerged a day or so longer in case anyone disagrees though.

Thank you for you contribution @samGreer!

[tacaswell]

Thank you @samGreer and congratulations on your first merged PR to Matplotlib 🎉

I hope we hear from you again.