Update initializer to use modern Ruby syntax

This trips over a Rubocop rule on CI:

```
lib/mail/notify/railtie.rb:12:36: W: Lint/SendWithMixinArgument: Use
prepend Mail::Notify::MailersController instead of send(:prepend,
Mail::Notify::MailersController).
          Rails::MailersController.send(:prepend,
Mail::Notify::MailersController)

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

As rubocop/rubocop#7274 explains:

> Back in the day (before Ruby 2.1), Module.include and Module.prepend
were private methods. So when people authored gems / patched monkeys,
they wrote e.g. Foo.send(:include, MyPatch). In our day and age, I
think we should encourage everybody to just write Foo.include(MyPatch).

lib/mail/notify/railtie.rb

@@ -9,7 +9,7 @@ class Railtie < Rails::Railtie
 
       initializer 'mail-notify.action_controller' do
         ActiveSupport.on_load(:action_controller, run_once: true) do
-          Rails::MailersController.send(:prepend, Mail::Notify::MailersController)
+          Rails::MailersController.prepend(Mail::Notify::MailersController)
         end
       end
     end