chore(tanstack-start): Update main path in WranglerConfig to src/server.ts to match example handler

[Friss]

Summary

The example handler is at src/server.ts but the config was pointing to app/server.ts which was confusing and wrong. It should point to the src file.

Screenshots (optional)

Documentation checklist

• The change adheres to the documentation style guide.

[MattieTK]

Thanks for noticing and fixing this!