update documentation on accessing new Mistral checkpoints on HF Hub

[David Hall]

Describe the bug

Currently README.md has incorrect directions for accessing checkpoints from the HF Hub. Per #86 they're on the hub as branches:

if you look at the different branches, you should see all 610 checkpoints (see here: https://huggingface.co/stanford-crfm/arwen-gpt2-medium-x21/tree/main).

but the readme has them as separate repos

git clone https://huggingface.co/stanford-crfm/arwen-x21-checkpoint-400000

@J38 is this what you meant in #109 ?

[J38]

This should be fixed now on main .