removed a dead link from fine-tuning.md in docs

[Riddhimaan-Senapati]

Description

Removed a dead link in the fine-tuning.md in docs as the file is no longer present in the repository. This is the link that was removed: https://docs.llamaindex.ai/en/stable/optimizing/examples/finetuning/gradient/gradient_fine_tuning.ipynb

Fixes #17619

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[Riddhimaan-Senapati]

@tobi @wpiekutowski @xevo @bborn @logan-markewich Can anyone of you review my PR?