doc: fix an output example in repl.md

Make `_` reassignment example match more with the current output.
Extend the example for more clarity.

PR-URL: https://github.com/nodejs/node/pull/10244


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>