Fixing incorrect arity in cross_context_boundaries.md (#6447)

Author: MaxLap

guides/data_modelling/cross_context_boundaries.md

@@ -316,7 +316,7 @@ defmodule HelloWeb.CartItemController do
 end
 ```
 
-We defined a new `CartItemController` with the create and delete actions that we declared in our router. For `create`, we call a `ShoppingCart.add_item_to_cart/2` function which we'll implement in a moment. If successful, we show a flash successful message and redirect to the cart show page; else, we show a flash error message and redirect to the cart show page. For `delete`, we'll call a `remove_item_from_cart` function which we'll implement on our `ShoppingCart` context  and then redirect back to the cart show page. We haven't implemented these two shopping cart functions yet, but notice how their names scream their intent: `add_item_to_cart` and `remove_item_from_cart` make it obvious what we are accomplishing here. It also allows us to spec out our web layer and context APIs without thinking about all the implementation details at once.
+We defined a new `CartItemController` with the create and delete actions that we declared in our router. For `create`, we call a `ShoppingCart.add_item_to_cart/3` function which we'll implement in a moment. If successful, we show a flash successful message and redirect to the cart show page; else, we show a flash error message and redirect to the cart show page. For `delete`, we'll call a `remove_item_from_cart` function which we'll implement on our `ShoppingCart` context  and then redirect back to the cart show page. We haven't implemented these two shopping cart functions yet, but notice how their names scream their intent: `add_item_to_cart` and `remove_item_from_cart` make it obvious what we are accomplishing here. It also allows us to spec out our web layer and context APIs without thinking about all the implementation details at once.
 
 Let's implement the new interface for the `ShoppingCart` context API in `lib/hello/shopping_cart.ex`: