docs: Delete redundant config in app router example

Author: amannn

examples/example-app-router/next.config.mjs

@@ -1,7 +1,11 @@
 import {NextConfig} from 'next';
 import createNextIntlPlugin from 'next-intl/plugin';
 
-const withNextIntl = createNextIntlPlugin();
+const withNextIntl = createNextIntlPlugin({
+  experimental: {
+    createMessagesDeclaration: './messages/en.json'
+  }
+});
 
 const config: NextConfig = {};