Update the API Reference link in Streamlit integration docs (#7377)
This page:
https://python.langchain.com/docs/modules/callbacks/integrations/streamlit
Has a bad API Reference link currently. This PR fixes it to the correct
link.
Also updates the embedded app link to
https://langchain-mrkl.streamlit.app/ (better name) which is hosted in
langchain-ai/streamlit-agent repo