docs: clarify LLM API configuration in mcp_simple_chatbot README

[ddaltn]

Motivation and Context

The mcp_simple_chatbot is preconfigured with a specific provider (Groq) and model. For clarity and to possibly save users time, it's helpful to reference this in the README.md. Otherwise, users may unintentionally use a different provider's API key resulting in failed usage of the example.

How Has This Been Tested?

All tests per the CONTRIBUTING.md were run and passed. With that said, this is a doc change that should have no impact on SDK functionality.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

n/a