Nit: Fix broken link in README.md

[redbrogdon]

There's a broken link in the README for dart_mcp_server pointing to this Url:

https://github.com/dart-lang/ai/tree/main/mcp_examples/bin/workflow_bot

[redbrogdon]

I thought workflow_bot might have been renamed to workflow_client, which is in that folder, but then I figured y'all would know the right thing to do and could fix it in ten seconds anyway. 😄