nixos/mosquitto: Fix instructions for password gen

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.

nixos/modules/services/networking/mosquitto.nix

@@ -125,8 +125,8 @@ in
               description = ''
                 Specifies the hashed password for the MQTT User.
                 <option>hashedPassword</option> overrides <option>password</option>.
-                To generate hashed password install <literal>mkpasswd</literal>
-                package and run <literal>mkpasswd -m sha-512</literal>.
+                To generate hashed password install <literal>mosquitto</literal>
+                package and use <literal>mosquitto_passwd</literal>.
               '';
             };