util: update comment in util.promisify

child_process.exec has Symbol('util.promisify.custom') in its keys
and no longer has Symbol('customPromisifyArgs') in its keys, but it was
still used as an example of funtions with
Symbol('customPromisifyArgs').

PR-URL: nodejs#25323
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>

lib/internal/util.js

@@ -282,7 +282,7 @@ function promisify(original) {
   }
 
   // Names to create an object from in case the callback receives multiple
-  // arguments, e.g. ['stdout', 'stderr'] for child_process.exec.
+  // arguments, e.g. ['bytesRead', 'buffer'] for fs.read.
   const argumentNames = original[kCustomPromisifyArgsSymbol];
 
   function fn(...args) {