unsafe: mutable-static: do not create reference (#2736)

`println!` adds references (&) to its arguments, to avoid moving them.

This is undesirable here, because it is extremely error-prone to take
references to `static mut`s. We could `println!("{}", {counter})`, but
this is somewhat exotic syntax and just sticking with `dbg!` also avoids
this problem as it does not add references.

src/unsafe-rust/mutable-static.md

@@ -32,7 +32,7 @@ fn main() {
 
     // SAFETY: There are no other threads which could be accessing `COUNTER`.
     unsafe {
-        println!("COUNTER: {COUNTER}");
+        dbg!(COUNTER);
     }
 }
 ```