nixos/sddm: add example for option sddm.theme (#418223)

nixos/modules/services/display-managers/sddm.nix

···
       theme = mkOption {
         type = types.str;
         default = "";
+        example = lib.literalExpression "\"\${pkgs.where-is-my-sddm-theme.override { variants = [ \"qt5\" ]; }}/share/sddm/themes/where_is_my_sddm_theme_qt5\"";
         description = ''
           Greeter theme to use.
         '';