Fix LLaMA2 dead link in README.md

[zhanwenchen]

The old deepcoda link is dead. The official LLaMA2 HF repo is working: https://huggingface.co/meta-llama/Llama-2-13b-hf/tree/main?clone=true

[Andy1621]

Hi! Thanks for your pull requests. In my opinion, LLaMA1 and LLaMA2 use different prompts, and replacing the link may not be suitable since we use the model based on LLaMA1.