Update the diagnostic of error[E0597] in dropck.md

[chansuke]

I was reading this page with playground(2018) and found that the diagnostic of error[E0597] of this example

struct Inspector<'a>(&'a u8);

impl<'a> Drop for Inspector<'a> {
    fn drop(&mut self) {
        println!("I was only {} days from retirement!", self.0);
    }
}

struct World<'a> {
    inspector: Option<Inspector<'a>>,
    days: Box<u8>,
}

fn main() {
    let mut world = World {
        inspector: None,
        days: Box::new(1),
    };
    world.inspector = Some(Inspector(&world.days));
    // Let's say `days` happens to get dropped first.
    // Then when Inspector is dropped, it will try to read free'd memory!
}

is slightly different with an output of the current version and it would be better if it can update its error message to the current one?

[JohnTitor]

Thanks!