docs(store): add missing types to walkthrough example (#3206)

Closes #3203

Author: muhammadawaisshaikh

projects/ngrx.io/content/examples/store-walkthrough/src/app/app.component.ts

@@ -17,11 +17,11 @@ export class AppComponent {
   books$ = this.store.select(selectBooks);
   bookCollection$ = this.store.select(selectBookCollection);
 
-  onAdd(bookId) {
+  onAdd(bookId: string) {
     this.store.dispatch(addBook({ bookId }));
   }
 
-  onRemove(bookId) {
+  onRemove(bookId: string) {
     this.store.dispatch(removeBook({ bookId }));
   }
 

projects/ngrx.io/content/examples/store-walkthrough/src/app/book-collection/book-collection.component.ts

@@ -8,5 +8,5 @@ import { Book } from '../book-list/books.model';
 })
 export class BookCollectionComponent {
   @Input() books: Array<Book>;
-  @Output() remove = new EventEmitter();
+  @Output() remove = new EventEmitter<string>();
 }

projects/ngrx.io/content/examples/store-walkthrough/src/app/book-list/book-list.component.ts

@@ -8,5 +8,5 @@ import { Book } from './books.model';
 })
 export class BookListComponent {
   @Input() books: Array<Book>;
-  @Output() add = new EventEmitter();
+  @Output() add = new EventEmitter<string>();
 }