Fix example code for unreachable!

The previous code was giving an incorrect result (not x/3).

Also, this function does not work with signed integers. It now accepts `u32` instead of `i32`.
No, with `<= 3*i`, this gives:

0/3=0
1/3=1
2/3=1
3/3=1
4/3=2
5/3=2
6/3=2