Update llama readme, use main branch for llama3

[pytorchbot]

Summary:
Use main branch for llama3. Looks like the warning was removed in D56358723 ?

See: #3850

Differential Revision: D58212855

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3871

• 📄 Preview Python docs built from this PR

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 0cd23b9 with merge base d3326a2 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (cmake, ic3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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