Update array key

pages/index.tsx

@@ -7,7 +7,7 @@ export default function Home(props: Props) {
     <ul>
       {props.books.map((book, idx) => {
         return (
-          <li key={`${idx}_${book.isbn ?? ""}`}>
+          <li key={`${idx}_${book.id}`}>
             {book.id}; {book.title}; {book.authors}; {book.isbn}
           </li>
         );