update schema url point to the actual location (#884)

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## Motivation and Context
This PR ensures that the draft URL points to the actual draft schema 
improvement suggested in
https://github.com/modelcontextprotocol/registry/issues/814


## How Has This Been Tested?
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tested? -->

## Breaking Changes
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## Types of changes
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the boxes that apply: -->
- [x] 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
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that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] 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
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docs/reference/server-json/CONTRIBUTING.md

@@ -26,5 +26,5 @@ When the draft changes are ready for release:
 
 ## Schema Versioning
 
-- **Draft schema**: `https://static.modelcontextprotocol.io/schemas/draft/server.schema.json` - For in-progress changes, may change without notice.
+- **Draft schema**: `https://raw.githubusercontent.com/modelcontextprotocol/registry/main/docs/reference/server-json/draft/server.schema.json` - For in-progress changes, may change without notice.
 - **Released schemas**: `https://static.modelcontextprotocol.io/schemas/YYYY-MM-DD/server.schema.json` - Stable, versioned by release date.