hi,
the email address for root@pam is in /etc/pve/user.cfg
the format is something like this: user:root@pam:1:0:::youremail@domain.com:::
you could replace this line with sed or similar, but before deploying the change test it on one machine to make sure user configuration doesn't break! (i suggest you take a backup of the user.cfg file beforehand to avoid issues)
even easier: pveum user modify root@pam --email yourmail@domain.com