It's a name of animation that has finished playing. In your case it would indeed be fade_in_out
.
You'll need another function to capture an argument that comes with signal. Signal's signature is this:
animation_finished ( String anim_name )
Try doing something like this:
onFadeInOutFinished( anim_name ):
goto_next_scene()
And connect it instead of goto_next_scene
like this:
$AnimationPlayer.connect("animation_finished", self, "onFadeInOutFinished")
You don't want to add and argument to goto_next_scene()
because it has no business knowing it's being called because some animation has stopped playing.