Ch. 2: intentionally use `{}` at first

This supports the prose in the guessing game tutorial, which uses this
as a way of teaching where you can do `{foo}` and where you cannot.

listings/ch02-guessing-game-tutorial/listing-02-01/src/main.rs

@@ -25,7 +25,7 @@ fn main() {
     // ANCHOR_END: expect
 
     // ANCHOR: print_guess
-    println!("You guessed: {guess}");
+    println!("You guessed: {}", guess);
     // ANCHOR_END: print_guess
 }
 // ANCHOR: all