fix #539: Change signature to trigger precise error message:

Now trigger this error:
```
error: expected type, found `)`
  --> exercises/functions/functions2.rs:10:16
   |
10 | fn call_me(num:) {
   |                ^ expected type

```

exercises/functions/functions2.rs

@@ -7,7 +7,7 @@ fn main() {
     call_me(3);
 }
 
-fn call_me(num) {
+fn call_me(num:) {
     for i in 0..num {
         println!("Ring! Call number {}", i + 1);
     }