docs(check_user_name): fix incoherent number

subjects/check_user_name/README.md

@@ -45,11 +45,11 @@ fn main() {
   let user0 = User::new("Didier".to_string(), AccessLevel::Admin);
   println!("{:?}", check_user_name(&user0));
 
-  let user2 = User::new("Mary".to_string(), AccessLevel::Normal);
-  println!("{:?}", check_user_name(&user2));
+  let user1 = User::new("Mary".to_string(), AccessLevel::Normal);
+  println!("{:?}", check_user_name(&user1));
 
-  let user3 = User::new("John".to_string(), AccessLevel::Guest);
-  println!("{:?}", check_user_name(&user3));
+  let user2 = User::new("John".to_string(), AccessLevel::Guest);
+  println!("{:?}", check_user_name(&user2));
 }
 ```