Fix `noticed:notifier` generator in README

[marcoroth]

Pull Request

Summary: Changes the Rails generator from noticed:notification to noticed:notifier in the README.

Related Issue: None

Description: Seems like this is a leftover.

Testing:

Running the noticed:notification throws an error.

❯ rails generate noticed:notification FirstSyncNotification
Could not find generator 'noticed:notification'. (Rails::Command::CorrectableNameError)
Did you mean?  noticed:notifier
Run `bin/rails generate --help` for more options.

Using the suggested noticed:notifier works:

❯ rails generate noticed:notifier FirstSyncNotification
      create  app/notifiers/first_sync_notification_notifier.rb

Screenshots (if applicable): -

Checklist:

• [x] Code follows the project's coding standards
• [ ] Tests have been added or updated to cover the changes
• [ ] Documentation has been updated (if applicable)
• [x] All existing tests pass
• [x] Conforms to the contributing guidelines

Additional Notes: None