fix: using rating() everywhere

src/app/books/book/book.component.html

@@ -1,16 +1,14 @@
-@let b = book();
-
 <div class="card mb-4">
   <div class="card-body bg-light">
     <img class="img-thumbnail float-end"
-      src="//api.angular.schule/avatar/{{ b.rating }}">
+      src="//api.angular.schule/avatar/{{ rating() }}">
     <div class="card-title">
       <h2>
         {{ title() }}
         <span class="badge bg-secondary me-2">{{ rating() }}</span>
       </h2>
     </div>
-    <p>{{ b.description }}</p>
+    <p>{{ book().description }}</p>
 
     <button [disabled]="rating() <= 1" class="btn btn-primary me-2" (click)="doRateDown()">-1</button>
     <button [disabled]="rating() >= 5" class="btn btn-primary" (click)="doRateUp()">+1</button>