Fix the path for openapi.yaml in README (#314)

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## Motivation and Context
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Noticed that the path for openapi.yaml was no longer working. This PR
fixes that.

## 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: -->
- [ ] 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)
- [x] 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/README.md

@@ -11,7 +11,7 @@ This extends [the main README](../README.md).
 
 ## API & Technical Specifications
 
-- [`openapi.yaml`](./openapi.yaml) - OpenAPI specification for the official registry API
+- [`openapi.yaml`](./server-registry-api/openapi.yaml) - OpenAPI specification for the official registry API
 - [`api_examples.md`](./api_examples.md) - Examples of what data will actually look like coming from the official registry API
 - [`architecture.md`](./architecture.md) - Technical architecture, deployment strategies, and data flows
 - [`server.json` README](./server-json/README.md) - description of the `server.json` purpose and schema