Fix warning

src/day11.rs

@@ -175,7 +175,7 @@ pub fn do_round(monkeys: &Vec<RefCell<Monkey>>, do_worry: bool) {
                 item,
                 target
             );
-            let mut target = &mut monkeys.get(target).unwrap().borrow_mut().items;
+            let target = &mut monkeys.get(target).unwrap().borrow_mut().items;
             target.push_back(item);
         }
     }