doc: updated YAML version representation in readline.md

All other versions in YAML throughout the docs start with _v_. Fix two
cases in `readline.md` that did not.

PR-URL: https://github.com/nodejs/node/pull/31924


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>