Add solutions to functions

exercises/02_functions/functions5.rs

@@ -1,8 +1,9 @@
-fn main() {
-    let answer = square(3);
-    println!("The square of 3 is {}", answer);
-}
-
+// TODO: Fix the function body without chaning the signature.
 fn square(num: i32) -> i32 {
     num * num;
 }
+
+fn main() {
+    let answer = square(3);
+    println!("The square of 3 is {answer}");
+}