Fix typo: shemars -> schemars in README

[floydremmerswaal]

Fix typo in README: "shemars" → "schemars"

Motivation and Context

Fixes a typo in the README where "shemars" should be "schemars" (the correct name of the JSON Schema library).

How Has This Been Tested?

N/A - Documentation-only change.

Breaking Changes

None.

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

Simple typo correction in the README's "Third Dependencies" section.