Code-Fehler bei Thema 06, Folie 19

Kasten rechts unten:
```rust
let b = Foo;
let c = &mut b;
```
> error[[E0596]](https://doc.rust-lang.org/stable/error-index.html#E0596): cannot borrow `b` as mutable, as it is not declared as mutable

Korrektur:
```rust
let mut b = Foo;
let c = &mut b;
```