Fix broken link to Chapter 13-01 in Chapter 12-04 (#2025)

Fix broken link to Chapter 13-01 in Chapter 12-04

src/ch12-04-testing-the-librarys-functionality.md

@@ -196,7 +196,7 @@ pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
 </span>
 
 The `lines` method returns an iterator. We’ll talk about iterators in depth in
-[Chapter 13][ch13]<!-- ignore -->, but recall that you saw this way of using an
+[Chapter 13][ch13-iterators]<!-- ignore -->, but recall that you saw this way of using an
 iterator in [Listing 3-5][ch3-iter]<!-- ignore -->, where we used a `for` loop
 with an iterator to run some code on each item in a collection.
 
@@ -266,7 +266,7 @@ At this point, we could consider opportunities for refactoring the
 implementation of the search function while keeping the tests passing to
 maintain the same functionality. The code in the search function isn’t too bad,
 but it doesn’t take advantage of some useful features of iterators. We’ll
-return to this example in [Chapter 13][ch13]<!-- ignore -->, where we’ll
+return to this example in [Chapter 13][ch13-iterators]<!-- ignore -->, where we’ll
 explore iterators in detail, and look at how to improve it.
 
 #### Using the `search` Function in the `run` Function
@@ -336,3 +336,4 @@ ch10-03-lifetime-syntax.html#validating-references-with-lifetimes
 [ch11-anatomy]: ch11-01-writing-tests.html#the-anatomy-of-a-test-function
 [ch10-lifetimes]: ch10-03-lifetime-syntax.html
 [ch3-iter]: ch03-05-control-flow.html#looping-through-a-collection-with-for
+[ch13-iterators]: ch13-02-iterators.html