doc: add example for setting Vary: Accept-Encoding header in zlib.md

PR-URL: nodejs#26308
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: mukulkhanna

doc/api/zlib.md

@@ -107,6 +107,8 @@ const http = require('http');
 const fs = require('fs');
 http.createServer((request, response) => {
   const raw = fs.createReadStream('index.html');
+  // Store both a compressed and an uncompressed version of the resource.
+  response.setHeader('Vary: Accept-Encoding');
   let acceptEncoding = request.headers['accept-encoding'];
   if (!acceptEncoding) {
     acceptEncoding = '';