Fix typo in active_record_basics.md [skip ci]

Author: yuler

guides/source/active_record_basics.md

@@ -250,7 +250,7 @@ purposes.
 Active Record models are placed under the `app/models` directory by default. But
 you may want to organize your models by placing similar models under their own
 folder and namespace. For example, `order.rb` and `review.rb` under
-`app/models/books` with `Book::Order` and `Book::Review` class names,
+`app/models/book` with `Book::Order` and `Book::Review` class names,
 respectively. You can create namespaced models with Active Record.
 
 In the case where the `Book` module does not already exist, the `generate`