test: adjust CLI flags test to ignore blank lines in doc

PR-URL: https://github.com/nodejs/node/pull/40403


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>