Update docs with new redirect helper

docs/advanced-features/preview-mode.md

@@ -111,8 +111,7 @@ export default async (req, res) => {
 
   // Redirect to the path from the fetched post
   // We don't redirect to req.query.slug as that might lead to open redirect vulnerabilities
-  res.writeHead(307, { Location: post.slug })
-  res.end()
+  res.redirect(post.slug)
 }
 ```