doc: var => let / const in repl.md

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>