[3.x] Fix Tween cheatsheet URL

[sk757a]

No description provided.

[AThousandShips]

Thank you for your work

Wrong link, it should be 3.6, this link:
"https://raw.githubusercontent.com/godotengine/godot-docs/3.6/img/tween_cheatsheet.png"
I'm not sure the 4.1 sheet applies correctly for the 3.x case, for example the "spring" mode doesn't exist in 3.x, only in 4.1:
3.6
4.1

[akien-mga]

I pushed an update to also do the change in SceneTreeTween docs.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉