Interrupt include not necessary on Pure Hw CTC Mode

Hi Dean,
I'm Testato, i follow you on AvrFreaks forum :-)

Your tutorial is always very good, i made 3 correction on Timer document, this is the first
15624 --> 15625 and 49911 --> 49910

This is the second patch,
You correctly write "We found the timer count to be 15625 for those conditions"
but after you use 15624 instead of xxxx5 then the following calculations are wrong, must
Merge pull request #7 from testato/patch-1

Remove unused `#include` from the pure HW CTC chapter.
Merge pull request #8 from testato/patch-2

Fix incorrect timer reload values.
Fix incorrect chapter references.
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?
Merge pull request #10 from testato/patch-3

Corrected incorrect chapter references.
Rebuild PDFs from latest source code.
Update PROGMEM tutorial with information on warnings produced by %S format strings in newer GCC versions.
Fix timer tutorial using the wrong variable type in the text.