Show mixed shared/exclusive reference errors

[jonathanpallant]

Old slides had this example: https://github.com/ferrous-systems/teaching-material/blob/main/presentations/ownership-borrowing-in-brief/5.rs

Some kind of example showing how mixed shared/exclusive references are not allowed would be useful.

[robamu]

I like the string example and the wording of the Rust book here: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

Users of an immutable reference don’t expect the value to suddenly change out from under them!

maybe we can add something similar?