feat: improved closure docs and exercises by using into_iter()

exercises/closures/closures5.rs

@@ -23,6 +23,7 @@ fn sum_letters(animals: &Vec<&str>) -> usize {
     // https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.fold
     // and the return types of the different ways of getting an iterator
     // https://doc.rust-lang.org/stable/std/iter/
+    // https://doc.rust-lang.org/std/iter/index.html#the-three-forms-of-iteration
     // https://doc.rust-lang.org/std/iter/trait.IntoIterator.html#tymethod.into_iter
     // TODO: change the next 2 lines to compile and pass the test.
     let sum_closure = |x: &usize, y: &usize| &(x + y);