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').
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines