Give a more structure hint for iteration exercises (#235)

exercises/06_ticket_management/05_iter/src/lib.rs

@@ -1,6 +1,10 @@
 use ticket_fields::{TicketDescription, TicketTitle};
 
 // TODO: Provide an `iter` method that returns an iterator over `&Ticket` items.
+//
+// Hint: just like in the previous exercise, you want to delegate the iteration to
+//   the `Vec<Ticket>` field in `TicketStore`. Look at the standard library documentation
+//   for `Vec` to find the right type to return from `iter`.
 #[derive(Clone)]
 pub struct TicketStore {
     tickets: Vec<Ticket>,