doc: update API reference URL in README.md (#578)

Update link in readme.md

## Motivation and Context

Kotlin SDK has moved from
https://modelcontextprotocol.github.io/kotlin-sdk/ to
https://kotlin.sdk.modelcontextprotocol.io/

-
[Announcement](https://discord.com/channels/1358869848138059966/1399986970851282954/1476078438266830931)
- DNS change PR: https://github.com/modelcontextprotocol/dns/pull/5

## Breaking Changes
No

## Types of changes

- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] 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
- [x] I have added or updated documentation as needed

README.md

@@ -887,7 +887,7 @@ allowing for easy testing of MCP functionality without the need for network setu
 
 ## Documentation
 
-- [API Reference](https://modelcontextprotocol.github.io/kotlin-sdk/)
+- [API Reference](https://kotlin.sdk.modelcontextprotocol.io/)
 - [Model Context Protocol documentation](https://modelcontextprotocol.io)
 - [MCP specification](https://modelcontextprotocol.io/specification/latest)