fix: keep main.tsx for JSX-containing code block in islands.md

Addresses review comment — the snippet contains JSX (<MyIsland />)
so the filename must be .tsx, not .ts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

docs/latest/concepts/islands.md

@@ -28,7 +28,7 @@ export default function MyIsland() {
 An island can be used anywhere like a regular Preact component. Fresh will take
 care of making it interactive on the client.
 
-```tsx main.ts
+```tsx main.tsx
 import { App, staticFiles } from "fresh";
 import MyIsland from "./islands/my-island.tsx";