docs: add missing import to `CartService` tutorial instructions (#42701)

PR Close #42701

aio/content/examples/getting-started/src/app/cart.service.ts

@@ -1,9 +1,9 @@
-// #docplaster
 // #docregion import-http
 import { Injectable } from '@angular/core';
 import { HttpClient } from '@angular/common/http';
+// #docregion props
 import { Product } from './products';
-// #enddocregion import-http
+// #enddocregion props, import-http
 
 @Injectable({
   providedIn: 'root'

aio/content/start/start-data.md

@@ -30,6 +30,7 @@ This section walks you through adding a **Buy** button and setting up a cart ser
 
     <code-example header="src/app/cart.service.ts" path="getting-started/src/app/cart.service.1.ts"></code-example>
 
+1. Import the `Product` interface from `./products.js`.
 1. In the `CartService` class, define an `items` property to store the array of the current products in the cart.
 
     <code-example path="getting-started/src/app/cart.service.ts" header="src/app/cart.service.ts" region="props"></code-example>