Node tutorial says freed instance references become null instead of invalid

**Your Godot version:**

Godot 4

**Issue description:**

In the `Nodes and scene instances` tutorial, the following line:

> You can alternatively call `free()` to immediately destroy the node. You should do this with care as any reference to it will instantly become `null`.

Should say "invalid" instead of "null". Saying that a reference becomes null means that it contains the `null` value, which is not what happens.

**URL to the documentation page:**

https://docs.godotengine.org/en/stable/tutorials/scripting/nodes_and_scene_instances.html#creating-nodes