fix: added typescript dependency in next-sitemap example (vercel#25866)

As mentioned in vercel#25854 `witn-next-sitemap` example is missing `typescript` package as a dependency

- [x] Make sure the linting passes

Author: vitalybaev

examples/with-next-sitemap/package.json

@@ -16,6 +16,7 @@
   "devDependencies": {
     "@types/node": "14.6.0",
     "@types/react": "^16.9.45",
-    "next-sitemap": "latest"
+    "next-sitemap": "latest",
+    "typescript": "^4.3.2"
   }
 }