Change fold to sum

src/fn/hof.md

@@ -36,7 +36,7 @@ fn main() {
         (0..).map(|n| n * n)                             // All natural numbers squared
              .take_while(|&n_squared| n_squared < upper) // Below upper limit
              .filter(|&n_squared| is_odd(n_squared))     // That are odd
-             .fold(0, |acc, n_squared| acc + n_squared); // Sum them
+             .sum();                                     // Sum them
     println!("functional style: {}", sum_of_squared_odd_numbers);
 }
 ```