nixos nix-daemon: lower priority is 19. Closes #8675

nixos/modules/services/misc/nix-daemon.nix

···
         default = 0;
         description = ''
           Nix daemon process priority. This priority propagates to build processes.
-          0 is the default Unix process priority, 20 is the lowest.
+          0 is the default Unix process priority, 19 is the lowest.
         '';
       };