Make errors on RefCounted.free() more accurate

[RandomShaper]

When attempting to call free() on a RefCounted object, GDScript complains that you have tried to free "a reference."

Reference was renamed to RefCounted in Godot 4. Therefore, the wording warrants an update, for consistency and clarity.

[dalexeev]

Please update it here too:

godot/core/object/object.cpp

Line 766 in 97b8ad1

ERR_FAIL_V_MSG(Variant(), "Can't 'free' a reference.");

[akien-mga]

Thanks!