Fix a value for disabling enqueue_after_transaction_commit [ci-skip]

`enqueue_after_transaction_commit` expects Symbol values, not Boolean.
https://github.com/rails/rails/blob/a472403d55ee5e654590e8597cb89acce928d9a9/activejob/lib/active_job/enqueue_after_transaction_commit.rb#L23-L30

Author: y-yagi

guides/source/7_2_release_notes.md

@@ -194,7 +194,7 @@ basis:
 
 ```ruby
 class NewTopicNotificationJob < ApplicationJob
-  self.enqueue_after_transaction_commit = false
+  self.enqueue_after_transaction_commit = :never
 end
 ```