doc: fix examples in repl.md

* Update an example according to an actual REPL session.
* Replace an arrow function with a common function to hold `this`.

PR-URL: https://github.com/nodejs/node/pull/12684


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>