doc: use _code name_ rather than _codename_

This is excrutiatingly minor, but every authoritative resource I've
checked (which is to say, a few dictionaries) uses "code name" primarily
or exclusively. Let's follow suit.

PR-URL: https://github.com/nodejs/node/pull/36611


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>