Inline variables printed with println! and friends (#315)

The course follows the style of inlining variable names where possible
in `println!` statements.

src/unsafe/mutable-static-variables.md

@@ -6,7 +6,7 @@ It is safe to read an immutable static variable:
 static HELLO_WORLD: &str = "Hello, world!";
 
 fn main() {
-    println!("HELLO_WORLD: {}", HELLO_WORLD);
+    println!("HELLO_WORLD: {HELLO_WORLD}");
 }
 ```
 
@@ -23,7 +23,7 @@ fn add_to_counter(inc: u32) {
 fn main() {
     add_to_counter(42);
 
-    unsafe { println!("COUNTER: {}", COUNTER); }  // Potential data race!
+    unsafe { println!("COUNTER: {COUNTER}"); }  // Potential data race!
 }
 ```