inline format arguments

In a few cases, removed the unneeded `&` - this causes a minor slowdown because compiler cannot eliminate those (yet).

assets/theme_preview.rs

@@ -1,5 +1,5 @@
   // Output the square of a number.
   fn print_square(num: f64) {
       let result = f64::powf(num, 2.0);
-      println!("The square of {:.2} is {:.2}.", num, result);
+      println!("The square of {num:.2} is {result:.2}.");
   }