docs: fix issues related to tutorial. (#24445)

PR Close #24445

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

@@ -1,5 +1,6 @@
 // #docregion , init
 import { InMemoryDbService } from 'angular-in-memory-web-api';
+import { Hero } from './hero';
 
 export class InMemoryDataService implements InMemoryDbService {
   createDb() {
@@ -17,4 +18,13 @@ export class InMemoryDataService implements InMemoryDbService {
     ];
     return {heroes};
   }
+
+  // Overrides the genId method to ensure that a hero always has an id.
+  // If the heroes array is empty,
+  // the method below returns the initial number (11).
+  // if the heroes array is not empty, the method below returns the highest
+  // hero id + 1.
+  genId(heroes: Hero[]): number {
+    return heroes.length > 0 ? Math.max(...heroes.map(hero => hero.id)) + 1 : 11;
+  }
 }