doc: fix macosx-firewall suggestion BUILDING

Adding back the changes made by commit# 791d560b, that suggests running
macosx-firewall.sh script after bulid step. These changes were deleted
by commit# fc102d01, but they are still applicable.

PR-URL: https://github.com/nodejs/node/pull/15829


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>