docs: fix singular table names in API guide's withAuthorInfo() method (#10058)

Co-authored-by: John Paul E Balandan <[email protected]>

Author: AlexRasch

user_guide_src/source/guides/api/code/013.php

@@ -16,7 +16,7 @@ class BookModel extends Model
     public function withAuthorInfo()
     {
         return $this
-            ->select('book.*, author.id as author_id, author.name as author_name')
-            ->join('author', 'book.author_id = author.id');
+            ->select('books.*, authors.name as author_name')
+            ->join('authors', 'books.author_id = authors.id');
     }
 }