Update 140-edit-mapping-books-author-book.mdx

rectify instructions

Author: wjteo

docs/140-edit-mapping-books-author-book.mdx

@@ -5,7 +5,7 @@ import Screenshot from "@site/src/components/Screenshot";
 - On the ERD, click `books`
 - On the relational mappings list on the right, click on the edit icon of `author_book`
 - Rename `Field name` to `authors`
-- Uncheck all fields
+- Rename `authorId` to `_id` and uncheck other fields
 - Click `Save and close`