updating help docs for hf-token arg in download.py

[SalmanMohammadi]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Please link to any issues this PR addresses.

Changelog

Removed doc referencing not supporting safetensors.

Test plan

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• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/991

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[RdoubleA]

This is a more accurate help message, thanks for addressing!