fix(event): fix poll expect message

[Valentin271]

The doc says

Returns Ok(true) if an Event is available otherwise it returns Ok(false).

So "no events available" would correspond to Ok(false).

The doc does not mention the Err case but looking at the internal code in crossterm::event::read.rs, this is the only place an error is returned.

        let event_source = match self.source.as_mut() {
            Some(source) => source,
            None => {
                return Err(std::io::Error::new(
                    std::io::ErrorKind::Other,
                    "Failed to initialize input reader",
                ))
            }
        };

[orhun]

Good catch, thanks!