doc: modernize and fix code examples in repl.md

* Improve UX in 2 code examples (add spaces between output and input
  for better readability).

* Replace indexOf() by startsWith().

PR-URL: https://github.com/nodejs/node/pull/12634


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