Lesson 17. Introduction to For Loops: Explain why we need to subtract 1 to board_size.x

In the second example of this section's lesson, `board_size.x - 1` is used as a parameter in the `range()` function when it should be `board_size.x` as `void range(length: int)` creates a list of numbers from 0 to length - 1.

This mistake can be seen again in the first practice lesson 'Using a for loop to move to the end of the board'. The user must recreate the mistake shown in this lesson's example to succeed in the practice lesson.