docs: fix incorrect link in expert level up

[durandtibo]

What does this PR do?

The link for "Level 24: Add a new accelerator or Strategy" in the expert skills page is not working and returns "404 page not found". This PR updates the filename of the "Level 24: Add a new accelerator or Strategy" file to be consistent, and update the link.

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📚 Documentation preview 📚: https://pytorch-lightning--20408.org.readthedocs.build/en/20408/

[lantiga]

Thanks!