docs: add missing instruction in HTTP section (#25715)

PR Close #25715

aio/content/examples/toh-pt6/src/app/in-memory-data.service.ts

@@ -1,7 +1,11 @@
 // #docregion , init
 import { InMemoryDbService } from 'angular-in-memory-web-api';
 import { Hero } from './hero';
+import { Injectable } from '@angular/core';
 
+@Injectable({
+  providedIn: 'root',
+})
 export class InMemoryDataService implements InMemoryDbService {
   createDb() {
     const heroes = [

aio/content/tutorial/toh-pt6.md

@@ -77,8 +77,13 @@ _after importing the `HttpClientModule`_,
 The `forRoot()` configuration method takes an `InMemoryDataService` class
 that primes the in-memory database.
 
-The _Tour of Heroes_ sample creates such a class 
-`src/app/in-memory-data.service.ts` which has the following content:
+The class `src/app/in-memory-data.service.ts` is generated by the following command:
+
+<code-example language="sh" class="code-shell">
+  ng generate service InMemoryData
+</code-example>
+
+This class has the following content:
 
 <code-example path="toh-pt6/src/app/in-memory-data.service.ts" region="init" header="src/app/in-memory-data.service.ts" linenums="false"></code-example>