cluster: fix comment regarding child_process file

Fixed comment for the description of INTERNAL_PREFIX in child_process
whereas the value is present in the internal/child_process.js.

PR-URL: https://github.com/nodejs/node/pull/39308


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>