Pluralize callback

Users could have more than one `before_destroy` callback in their models.

[ci skip]

guides/source/active_record_callbacks.md

@@ -117,7 +117,9 @@ Here is a list with all the available Active Record callbacks, listed in the sam
 
 WARNING. `after_save` runs both on create and update, but always _after_ the more specific callbacks `after_create` and `after_update`, no matter the order in which the macro calls were executed.
 
-NOTE: `before_destroy` callback should be placed before `dependent: :destroy` associations (or use the `prepend: true` option), to ensure they execute before the records are deleted by `dependent: :destroy`.
+NOTE: `before_destroy` callbacks should be placed before `dependent: :destroy`
+associations (or use the `prepend: true` option), to ensure they execute before
+the records are deleted by `dependent: :destroy`.
 
 ### `after_initialize` and `after_find`