docs(isr): add missing key prop in jsx loop (#33984)

Copy pasting this example with a default create-next-app leads to an eslint warning. So I thought let's fix this.

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

docs/basic-features/data-fetching/incremental-static-regeneration.md

@@ -31,7 +31,7 @@ function Blog({ posts }) {
   return (
     <ul>
       {posts.map((post) => (
-        <li>{post.title}</li>
+        <li key={post.id}>{post.title}</li>
       ))}
     </ul>
   )