Fix the documentation (saving games) for godot 3

Sorry if that sounds like duplicated from:
https://github.com/godotengine/godot/issues/9984
but i realized i had posted at the wrong place, here is the correct place to post this issue.

Operating system or device - Godot version:
Ubuntu 14.04 Godot 3.0 alpha

I was following this tutorial:
http://docs.godotengine.org/en/latest/learning/features/misc/saving_games.html
and got this error:
Invalid call. Nonexistent function 'to_json' in base 'Dictionary'.
i took me a while to figure out that insted of:

savegame.store_line(nodedata.to_json())
i should do that:
savegame.store_line(to_json(nodedata))