nixos-option: don't eval description

see https://github.com/NixOS/nixpkgs/pull/31630#issuecomment-344073438

Changed files
+1 -1
nixos
modules
installer
+1 -1
nixos/modules/installer/tools/nixos-option.sh
···
fi
echo "Description:"
echo
-
eval printf $(evalOpt "description")
+
echo $(evalOpt "description")
echo $desc;