doc: fix a code example in zlib.md

PR-URL: nodejs#31264
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/zlib.md

@@ -115,7 +115,7 @@ 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');
+  response.setHeader('Vary', 'Accept-Encoding');
   let acceptEncoding = request.headers['accept-encoding'];
   if (!acceptEncoding) {
     acceptEncoding = '';