nixos/hamster: fix programs.hamster.enable text
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/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs