Correct 'McpTool' to 'McpServerTool' in README

[asklar]

Fix README to say McpServerTool, not McpTool to match the code.

Motivation and Context

doc bug

How Has This Been Tested?

n/a

Breaking Changes

n/a

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

[eiriktsarpalis]

Thanks. FYI @MackinnonBuck this might require a rebase of your changes.