Fix prompt for channel conversation summarization

[djjimba]

Type of change (place an x in the [ ] that applies)

• New sample
• New feature
• Bug fix
• Documentation

Summary

My initial intent was to fix typos conversaton, and cath in the prompt for channel conversation summarization,
but went ahead and updated it overall without changing the intent of the prompt.

Requirements (place an x in each [ ] that applies)

• I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

[zimeg]

Hey @djjimba! 👋 Sharp eye with this PR, it's always appreciated 👁️‍🗨️

I noted one nuance of Slack API wording, but that's a small change so let's merge this after for a nice improvement 🎉