docs: Fix broken link to Planets example on ReadMe

[deadlydog]

PR Summary

This is a small PR to fix a broken link to the Planets Example on the ReadMe. It currently has a link to the old location before it was moved into the docs directory, and so it gives a 404 error when you navigate to it at https://github.com/pester/Pester/blob/main/Examples/Planets.

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• [ x] Tests are added/update (if required)
• [ x] Documentation is updated/added (if required)

[deadlydog]

Oh, looks like this same fix is already in PR #2102 , so I'll close this one.