Clarify McpClientTool inherits from AIFunction in README.md

[ferrywlto]

Fix a typo McpClientTool written as AIFunction, which seems didn't update from the mcpdotnet repository.

Motivation and Context

Makes the documentation clear for beginners.

How Has This Been Tested?

This is documentation updating a typo which cannot be tested.

Breaking Changes

Nope

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