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.

Changed files
+2 -2
nixos
modules
services
networking
+2 -2
nixos/modules/services/networking/mosquitto.nix
···
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>.
'';
};
···
description = ''
Specifies the hashed password for the MQTT User.
<option>hashedPassword</option> overrides <option>password</option>.
+
To generate hashed password install <literal>mosquitto</literal>
+
package and use <literal>mosquitto_passwd</literal>.
'';
};