doc: white space unification in repl.md

Add an infix space in an argument list.
Change `>` into `> ` in code bits and output examples.
Explicitly clarify that default REPL prompt contains a trailing space.

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>