Fixed compiler warnings.

src/tasks/day13.rs

@@ -71,7 +71,7 @@ fn perform_round(map: &[Vec<char>], carts: &mut Vec<Cart>) {
         .collect();
 
     for cart_index in 0..carts.len() {
-        let mut cart = &mut carts[cart_index];
+        let cart = &mut carts[cart_index];
         if !cart.active {
             continue;
         }