Update the doc to use breakpoint()

[briannnyee]

According to my friend ChatGPT, using pdb.set_trace() is too old-school. Update the doc to use breakpoint() instead to stay young 😎

What does this PR do?

Update the doc to use breakpoint()

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📚 Documentation preview 📚: https://pytorch-lightning--20691.org.readthedocs.build/en/20691/