docs: note about a silly edge case in the ssr guide (#18628)

Author: andriibeee

Diff for: docs/guide/ssr.md

@@ -149,7 +149,7 @@ app.use('*', async (req, res, next) => {
     const appHtml = await render(url)
 
     // 5. Inject the app-rendered HTML into the template.
-    const html = template.replace(`<!--ssr-outlet-->`, appHtml)
+    const html = template.replace(`<!--ssr-outlet-->`, () => appHtml)
 
     // 6. Send the rendered HTML back.
     res.status(200).set({ 'Content-Type': 'text/html' }).end(html)