doc: modernize and fix code examples in process.md

* Replace `var` by `const`.
* Use `console.error()`.
* Fix example file name mismatch.
* Update output examples.
* Fix output readability
  (add a line break, remove an unnecessary duplicate word).

PR-URL: https://github.com/nodejs/node/pull/12381


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>