nixpkgs module: update config example

firefox.enableGeckoMediaPlayer does not exist anymore.

nixos/modules/misc/nixpkgs.nix

···
       default = {};
       example = literalExample
         ''
-          { firefox.enableGeckoMediaPlayer = true; }
+          { allowBroken = true; allowUnfree = true; }
         '';
       type = configType;
       description = ''