nixos/soft-serve: fix typos in option descriptions

nixos/modules/services/misc/soft-serve.nix

···
 {
   options = {
     services.soft-serve = {
-      enable = mkEnableOption "Enable soft-serve service";
+      enable = mkEnableOption "soft-serve";
 
       package = mkPackageOption pkgs "soft-serve" { };
 
···
         type = format.type;
         default = { };
         description = mdDoc ''
-          The contents of the configuration file.
+          The contents of the configuration file for soft-serve.
 
           See <${docUrl}>.
         '';