switch pepy to pypi downalod badge

[Borda]

What does this PR do?

Resolves #18340 (comment)
the pepy badge was recently unstable and often returned 404 so this is a replacement that does not have total downloads, only per the last period...

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cc @Borda @carmocca @justusschock @awaelchli

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 link-check

Check ID	Status
check-md-links / markdown-link-check	success	✅

These checks are required after the changes to README.md, src/lightning_app/README.md, src/lightning_fabric/README.md, src/pytorch_lightning/README.md.

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[Borda]

Ref: psincraian/pepy#573