Fix URLs in NeptuneLogger docstring

[aknierim]

What does this PR do?

Neptune updated their API and docs to version 3.X. This small PR fixes the URLs in the NeptuneLogger docstring to point to the correct version 2.X legacy docs.

Fixes #21371

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📚 Documentation preview 📚: https://pytorch-lightning--21372.org.readthedocs.build/en/21372/

[deependujha]

closing PR. Please check comment