Uneasy about altering code for shopping cart….

I have a new product designer plugin that I have installed on my site. I am using the Highend Child theme. The product designer is connected to woocommerce. When a (designed) product is added to the shopping cart, a thumbnail image of the customized product should appear in the cart at checkout. However, it does not do that.

I contacted the company who makes the plugin and was sent this message:

The cart.php in the woocommerce templates of your theme does not include the required:
do_action( ‘woocommerce_cart_item_thumbnail’ );
Check out the standard template in plugins/woocommerce/templates/cart/cart.php, to see how it is supposed to look.

<td class=”product-thumbnail”>
<?php
$thumbnail = apply_filters( ‘woocommerce_cart_item_thumbnail’, $_product->get_image(), $cart_item, $cart_item_key );

if ( ! $_product->is_visible() )
echo $thumbnail;
else
printf( ‘%s‘, $_product->get_permalink(), $thumbnail );
?>
</td>

They will not provide any more advice because they said this is a theme issue, not a plugin issue. My question is that I do not know what to do with the code they recommended I add:
do_action( ‘woocommerce_cart_item_thumbnail’ );
Does that go in the Child theme? If so, do I create a folder for it? And where do I place it in the code?

Thank you so much for your patience, time, and help!