Fix llama 2 urls in README.md

[upura]

I found that llama 2 urls in the leaderboard return 404. I checked the commit d4bdbfa and updated the following urls:

• https://huggingface.co/meta-llama/Llama-2-7b-hf
• https://huggingface.co/meta-llama/Llama-2-7b-chat-hf
• https://huggingface.co/meta-llama/Llama-2-13b-hf
• https://huggingface.co/meta-llama/Llama-2-13b-chat-hf