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

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

src/generics/trait-objects.md

@@ -6,7 +6,7 @@ We've seen how a function can take arguments which implement a trait:
 use std::fmt::Display;
 
 fn print<T: Display>(x: T) {
-    println!("Your value: {}", x);
+    println!("Your value: {x}");
 }
 
 fn main() {