Fix new clippy warnings


Rust 1.67 introduced a new clippy warning, a few of the exercises are affected. The warning is only related to idiomatic format strings, nothing major.

I opened a pull request with the fixes. It would have to be reopened if someone would like to review it.

Cheers, Remo

Thanks! For some reason I can’t reopen this. I’ll ask Katrina and Erik to see if they have ideas why.