Many redirects target a non existent slide

[bridadan]

In book.toml, many redirects target a non existent slide.

For example, https://google.github.io/comprehensive-rust/async/control-flow/select.html redirects to https://google.github.io/comprehensive-rust/async/control-flow/select.html, which is also incorrect.

I propose fixing the redirects and then adding a CI check to ensure all redirects remain valid.