fix analytics plugin output

plugins/analytics.js

@@ -2,6 +2,6 @@ module.exports = {
   onPageRendered: async ({ html, file }) => {
     // Example: inject analytics script into each page
     const snippet = '\n<script>console.log("Page viewed: ' + file + '")</script>';
-    return html.replace('</body>', `${snippet}</body>`);
+    return { html: html.replace('</body>', `${snippet}</body>`) };
   }
 };