doc: fix a function name in repl.md

`eval` => `myEval` to not shadow the global `eval`

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>