change ts template readme api/index file extension from tsx to ts (ve…

…rcel#27485)

## Documentation / Examples

- [x] Make sure the linting passes

### Changes
* Corrected the file extension reference from .tsx to .ts for ```packages/create-next-app/templates/typescript/README-template.md```. 
This matches the actual file extension and avoids confusion for new users.

packages/create-next-app/templates/typescript/README-template.md

@@ -14,7 +14,7 @@ Open [http://localhost:3000](http://localhost:3000) with your browser to see the
 
 You can start editing the page by modifying `pages/index.tsx`. The page auto-updates as you edit the file.
 
-[API routes](https://nextjs.org/docs/api-routes/introduction) can be accessed on [http://localhost:3000/api/hello](http://localhost:3000/api/hello). This endpoint can be edited in `pages/api/hello.tsx`.
+[API routes](https://nextjs.org/docs/api-routes/introduction) can be accessed on [http://localhost:3000/api/hello](http://localhost:3000/api/hello). This endpoint can be edited in `pages/api/hello.ts`.
 
 The `pages/api` directory is mapped to `/api/*`. Files in this directory are treated as [API routes](https://nextjs.org/docs/api-routes/introduction) instead of React pages.