Fix visible "ANCHOR: here" in listing 13-21

[obi1kenobi]

Listing 13-21 in Chapter 13.2 is currently showing ANCHOR: here and ANCHOR_END: here markers, perhaps unintentionally so: https://doc.rust-lang.org/book/ch13-02-iterators.html

struct Counter {
    count: u32,
}

impl Counter {
    fn new() -> Counter {
        Counter { count: 0 }
    }
}

// ANCHOR: here
impl Iterator for Counter {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        if self.count < 5 {
            self.count += 1;
            Some(self.count)
        } else {
            None
        }
    }
}
// ANCHOR_END: here

I'm not familiar with the #rustdoc_include semantics, so this is my attempt to suggest a fix based on how the neighboring listings appear to be set up.

[carols10cents]

Whoops, great catch! I messed this up in eb60fed, and adding the here anchor to the rustdoc_include is the right fix 👍 Thank you!

[obi1kenobi]

Thanks for the review and for the awesome Rust book content!