Saving Games page incorrect JSON functions in example

[ztc0611]

Your Godot version: 4.0 Alpha 14

Issue description: The docs page for Saving Games still uses the to_json() and parse_json() syntax which is now invalid in Godot 4.0.

URL to the documentation page: https://docs.godotengine.org/en/latest/tutorials/io/saving_games.html?highlight=serialization#saving-and-reading-data