Chapter 3 - Section Associated Types - &mut Counter is not an iterator

[glafrance]

In Chapter 3, in the section Associated Types, the provided sample code does not compile. In the main() function "for" loop you get this error:

for number in &mut counter {
   |                   ^^^^^^^^^^^^ `&mut Counter` is not an iterator

The provided code on this github repository also has the same error. Note that the provided code on this github repo does not have a main() function.