Actually print something in prime_squares example (#1271)

As suggested in #1268.

src/traits/from-iterator.md

@@ -9,6 +9,7 @@ fn main() {
         .into_iter()
         .map(|prime| prime * prime)
         .collect::<Vec<_>>();
+    println!("prime_squares: {prime_squares:?}");
 }
 ```