doc: clarify macosx-firewall suggestion BUILDING

`./tools/macosx-firewall.sh` fails if run before build step. Since the
suggestion comes before the build steps in the document, this change
clarifies that the script should be run after building.

PR-URL: https://github.com/nodejs/node/pull/10311


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>