Add solutions to functions

exercises/02_functions/functions3.rs

@@ -1,9 +1,10 @@
-fn main() {
-    call_me();
-}
-
 fn call_me(num: u32) {
     for i in 0..num {
         println!("Ring! Call number {}", i + 1);
     }
 }
+
+fn main() {
+    // TODO: Fix the function call.
+    call_me();
+}