[Docs] Fix broken list rendering of Instrumentation in the README

[koic]

Motivation and Context

[Docs] Fix the broken rendering list of Instrumentation in the README

This PR fixes the broken rendering of the following list: https://github.com/modelcontextprotocol/ruby-sdk/tree/v0.2.0?tab=readme-ov-file#instrumentation

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