Corrected four Chapter 3 reference

Hi Dean, 
i seen that you adopt my suggestion on Chapter reference correction and close my Issue https://github.com/abcminiuser/avr-tutorials/issues/9

Why did not you correct the four references to chapters 3? 

If you control, all four times that you re-link to Chapter 2, you speak about a formula with the Prescale, and the Prescale is explained in chapter 3, not in 2. 

Maybe I misunderstood something?