Close rust-lang#1464

Adds a `main` function to the last impl Trait example that demonstrates the expected behavior.

src/trait/impl_trait.md

@@ -68,4 +68,10 @@ fn double_positives<'a>(numbers: &'a Vec<i32>) -> impl Iterator<Item = i32> + 'a
         .filter(|x| x > &&0)
         .map(|x| x * 2)
 }
+
+fn main() {
+    let singles = vec![-3, -2, 2, 3];
+    let doubles = double_positives(&singles);
+    assert_eq!(doubles.collect::<Vec<i32>>(), vec![4, 6]);
+}
 ```