doc: fix missing argument for dns.resolvePtr()

PR-URL: https://github.com/nodejs/node/pull/12256
Fixes: https://github.com/nodejs/node/issues/12161


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>