Use dbg! instead of println! in day 3 & 4. (#2669)

Part of #2478 to clean up code blocks when all that is needed is a
trivial debug print statement.

Co-authored-by: Eric Githinji <egithinji@google.com>

src/smart-pointers/rc.md

@@ -14,8 +14,8 @@ fn main() {
     let a = Rc::new(10);
     let b = Rc::clone(&a);
 
-    println!("a: {a}");
-    println!("b: {b}");
+    dbg!(a);
+    dbg!(b);
 }
 ```