nixos/hamster: fix programs.hamster.enable text

mkEnableOption already adds "Whether to enable" and ends with a ".", so
remove that duplication from the help text.

Also reword it slightly while at it.

nixos/modules/programs/hamster.nix

@@ -6,7 +6,7 @@ with lib;
   meta.maintainers = pkgs.hamster.meta.maintainers;
 
   options.programs.hamster.enable =
-    mkEnableOption "Whether to enable hamster time tracking.";
+    mkEnableOption "hamster, a time tracking program";
 
   config = lib.mkIf config.programs.hamster.enable {
     environment.systemPackages = [ pkgs.hamster ];