fix: fix a mistake

src/advance/smart-pointer/rc-arc.md

@@ -64,7 +64,7 @@ fn main() {
         let a = Rc::new(String::from("test ref counting"));
         println!("count after creating a = {}", Rc::strong_count(&a));
         let b =  Rc::clone(&a);
-        println!("count after creating b = {}", Rc::strong_count(&a));
+        println!("count after creating b = {}", Rc::strong_count(&b));
         {
             let c =  Rc::clone(&a);
             println!("count after creating c = {}", Rc::strong_count(&c));