fix(functions3): improve function argument type (rust-lang#687)

Author: flakolefluk

exercises/functions/functions3.rs

@@ -7,7 +7,7 @@ fn main() {
     call_me();
 }
 
-fn call_me(num: i32) {
+fn call_me(num: u32) {
     for i in 0..num {
         println!("Ring! Call number {}", i + 1);
     }