L17.P1: rework and clarify practice

[NathanLovato]

The practice lacks documentation and mention of the cell variable outside of hints.

Example of code that learners might try to write:

func move_to_bottom():  
    for cell in range(board_size.y - 1):  
        position = Vector2(0,cell.y)  

func move_to_bottom():
	for cell in range(board_size.y - 1):
		position = Vector2(0,cell.y)

This also leads them to an error message that's not covered:

cannot find member y in base int.