add missing ; to return ()

[kalmarek]

Note that all other remaining test functions do suppress return value with an explicit semicolon. However these two don't and the following (I guess still valid?) solution

    pub fn set_title(&mut self, title: String) -> &Self {
        if title.is_empty() {
            panic!("Title cannot be empty");
        } else if title.len() > 50 {
            panic!("Title cannot be longer than 50 bytes");
        }
        self.title = title;
        return self
    }

throws an error:

   Compiling setters v0.1.0 (/home/kalmar/local/rust/100-exercises-to-learn-rust/exercises/03_ticket_v1/07_setters)
error[E0308]: mismatched types
   --> exercises/03_ticket_v1/07_setters/src/lib.rs:110:9
    |
110 | /         Ticket::new(valid_title(), valid_description(), "To-Do".into())
111 | |             .set_title(overly_long_title())
    | |___________________________________________^ expected `()`, found `&Ticket`
    |
help: consider using a semicolon here
    |
111 |             .set_title(overly_long_title());
    |                                            +
help: try adding a return type
    |
109 |     fn title_cannot_be_longer_than_fifty_chars() -> &Ticket {
    |                                                  ++++++++++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `setters` (lib test) due to 1 previous error

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[LukeMathWalker]

The style recommended in the corresponding chapter returns (), so I don't think the test should allow for a different solution when going for the &mut self approach.

[hdoordt]

All right, I missed the style recommendation @LukeMathWalker refers to, and so I agree that we should not allow returning anything other than () in the tests. Closing this PR