[doc fix] Fix broken links to PyTorch Lightning Bolts in pl_examples/README.md

[danielykim]

What does this PR do?

This PR is to fix broken links to PyTorch Lightning Bolts in pl_examples/README.md.

• Broken link: https://lightning-bolts.readthedocs.io/en/latest/convolutional.html#gpt-2
• Fixed link: https://pytorch-lightning.readthedocs.io/en/latest/ecosystem/bolts.html
  • Reviewers may suggest more appropriate links

Does your PR introduce any breaking changes? If yes, please list them.

None

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Make sure you had fun coding 🙃

[SeanNaren]

Thanks! Bolts is going through a refresh, so will make a followup issue to update this page.

[tchaton]

LGMT !