Update 20-edit-mapping-books-book-genre.mdx

the screenshot and description of the activity call for changing genres.genre to genres.name.

Author: mrlynn

docs/80-edit-mapping-rules/20-edit-mapping-books-book-genre.mdx

@@ -34,6 +34,7 @@ genres: [
 - Click on the edit icon of the `book_genre` embedded array.
 - Change the field name from `bookGenres` to `genres`.
 - Uncheck `bookId` and `genreId`.
+- Change the fieldname from `genre` to `name`
 - Click `Save and close`.