Fix: fixed error messages in lesson 20 part 2 (#1146)

* Fix: Fixed incorrect error messages in lesson 20

* Fix: Fixed lesson 20 error messages

* c

---------

Co-authored-by: Nathan Lovato <12694995+NathanLovato@users.noreply.github.com>

Author: mstphayassin

game_demos/DrawingTurtle.gd

@@ -59,11 +59,16 @@ func move_forward(distance: float) -> void:
 		previous_point = _points[-1]
 	var new_point := previous_point + Vector2.RIGHT.rotated(deg2rad(turn_degrees)) * distance
 	new_point = new_point.snapped(Vector2.ONE)
+	var is_closed := false
+	# Consider the polygon closed if a coordinate is repeated.
+	if new_point in _points:
+		is_closed = true
 	_points.append(new_point)
-
 	_temp_command_stack.append(
 		{command = "move_to", target = new_point + position + _current_offset}
 	)
+	if is_closed:
+		_close_polygon()
 
 
 func turn_right(angle_degrees: float) -> void: