docs: update README external links

[npellegrin]

Fix links in the README file.

Motivation and Context

It appears that the domain spec.modelcontextprotocol.io is no longer valid, the documentation is now at modelcontextprotocol.io. Also, the documentation points to a version of the specification that no longer seems to be aligned with the current implementation.

How Has This Been Tested?

N/A - documentation update

Breaking Changes

No breaking change.

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

This is my first PR here, please feel free to comment on the commit format or make any other suggestions :)