README: JSON mode link points to repo root instead of specific file

[giwaov]

Summary

The JSON mode section states:

standalone script jsonmode.py available here: https://github.com/NousResearch/Hermes-Function-Calling/tree/main

This links to the repository root directory rather than the specific jsonmode.py file. Should link directly to:

https://github.com/NousResearch/Hermes-Function-Calling/blob/main/jsonmode.py

(or wherever the file is located in the repo)