slurm service: fix typo in option description

nixos/modules/services/computing/slurm/slurm.nix

···
         defaultText = "pkgs.slurm-llnl";
         example = literalExample "pkgs.slurm-llnl-full";
         description = ''
-          The packge to use for slurm binaries.
+          The package to use for slurm binaries.
         '';
       };