Minor issues in the book quickstart

[robertpateii]

Regarding https://cucumber-rs.github.io/cucumber/current/quickstart.html

The path in main for the quickstart guide does not match the instructions given in the text. To fix, quickstart/concurrent and quickstart/simple should both be replaced by animal in the main function code.

Separately, under the Asyncness heading the code does not mention you need to add another use statement like the previous code does. So this should be added to it:
// Don't forget to additionally use tokio::time::{sleep, Duration};

I'll do a PR for this tomorrow.

[tyranron]

@robertpateii 🙏

[robertpateii]

@tyranron PR is up. thanks! good quickstart!