docs: Correct var reference in prerender example (#7434)

Author: rschristian

src/content/reference/react-dom/static/prerender.md

@@ -230,7 +230,7 @@ async function renderToString() {
     bootstrapScripts: ['/main.js']
   });
 
-  const reader = stream.getReader();
+  const reader = prelude.getReader();
   let content = '';
   while (true) {
     const {done, value} = await reader.read();