Correct subject for doubtful

subjects/doubtful/README.md

@@ -2,14 +2,20 @@
 
 ### Instructions
 
-Write a function called `doubtful` that adds to every string passed to it a question mark (?)
+Write a function called `doubtful` that adds to every string passed to it a question mark (?).
 
-You have to fix the code to make it compile an for that you can only modify the code where is indicated
+You have to complete the signature the functions to make it fit the usage and you also have to modify the usage to be able to do what is expected.
+
+- Restrictions:
+
+  - `doubtful` cannot return any value.
+
+  - And in the example of the usage you can only modify the argument of the function
 
 ### Expected
 
 ```rust
-pub fn doubtful(s: &mut String) {
+pub fn doubtful(s: /*give the correct type*/ ) {
 }
 ```
 
@@ -22,7 +28,9 @@ fn main() {
 	let mut s = String::from("Hello");
 
 	println!("Before changing the string: {}", s);
-	doubtful(&mut s);
+
+	doubtful(/*add your code here*/);
+
 	println!("After changing the string: {}", s);
 }
 ```