removing author_id

changed author_id to author since we haven't introduced the author model at the time we share this solution.

08-rails/code_samples/update_controller_test.rb

@@ -21,7 +21,7 @@
 
     book = Book.find_by(id: id)
     expect(book.title).must_equal book_hash[:book][:title]
-    expect(book.author_id).must_equal book_hash[:book][:author_id]
+    expect(book.author).must_equal book_hash[:book][:author]
     expect(book.description).must_equal book_hash[:book][:description]
   end
 
@@ -48,7 +48,7 @@
 
     book = Book.find_by(id: id)
     expect(book.title).must_equal original_book.title
-    expect(book.author_id).must_equal original_book.author_id
+    expect(book.author).must_equal original_book.author
     expect(book.description).must_equal original_book.description
   end
 end