update readme.md - add command to download llama3

[lessw2020]

Context

This PR adds the download instructions to pull the llama3 8B weights. Currently the readme talks about llama3 but then only has the download info to get llama2 weights. Took a few minutes to figure out how to update to pull, so figured better to update the readme and save users the hassle.

Changelog

only changes readme.md

Test plan

used the command in the readme to pull llama3 weights.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/852

• :page_facing_up: Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

:x: 2 New Failures

As of commit 55fb713493f88f607976d8ea83ef499ea0fb4bb0 with merge base bec7babec9c924a0ee7ad27e3f6582bc5bd1fef5 ():

NEW FAILURES - The following jobs have failed:

• Multi-GPU Recipe Tests / recipe_test_multi_gpu (3.8, stable) (gh) tests/recipes/test_eleuther_eval.py::TestEleutherEval::test_torchtune_checkpoint_eval_results
• Multi-GPU Recipe Tests / recipe_test_multi_gpu (3.9, stable) (gh) ##[error]The operation was canceled.

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