added url endpoints for book and author list and detail views

bookshelf/urls.py

@@ -6,5 +6,5 @@ urlpatterns = [
     path('books/', BookListView.as_view()),
     path('books/<int:pk>/', BookDetailView.as_view()),
     path('author/', AuthorListView.as_view()),
-    path('author/<int:pk>/', AuthorDetailView),
+    path('author/<int:pk>/', AuthorDetailView()),
 ]
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