Fix extension to tsx (tauri-apps#964)

Author: tsukinose81

docs/guides/getting-started/setup/next-js.mdx

@@ -152,7 +152,7 @@ To call our newly created command we will use the [`@tauri-apps/api`] JavaScript
 
 <InstallTauriApi />
 
-With the library installed, you can modify your `pages/index.ts` file to call the Command:
+With the library installed, you can modify your `pages/index.tsx` file to call the Command:
 
 ```typescript title=pages/index.tsx
 import { invoke } from '@tauri-apps/api/tauri'