Fixes https://github.com/NixOS/nixpkgs/issues/27996.
Updates instructions for generating hashes passwords for use in a
Mosquitto password file. Using `mosquitto_passwd` to generate these
hashes is a little less convenient, but the results are more likely to
be compatible with the mosquitto daemon.
As far as I can tell, the hashes generated with `mkpassd` did not work
as intended. But this may have been hidden by another bug:
https://github.com/NixOS/nixpkgs/issues/27130.
