Update README to reference server.tsx file (#646)

File reference has changed from `server.ts` -> `server.tsx`.

Author: msutkowski

guides/anthropic-patterns/README.md

@@ -121,7 +121,7 @@ state: {
 };
 ```
 
-Read more in the [source code](src/server.ts).
+Read more in the [source code](src/server.tsx).
 
 ## Why Durable Objects?