Update LLM_finetuning.md

[varunfb]

Added missing link to learning more about FSDP and getting started with FSDP.

What does this PR do?

Updated the documentation for FSDP.

Fixes # (issue)
N/A

Feature/Issue validation/testing

Verified by looking at the preview of the file.

Before submitting

• [X ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Thanks for contributing 🎉!

[HamidShojanazeri]

@varunfb can you pls make sure spell check is passing as well. Word list can be found here.

[mreso]

LGTM