doc: update instructions for cc

- update intructions on adding cc for email to
  nodejs-sec mailing list as UI does not allow it
  to be done directly.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/39674


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>