By default 'Hello!' or 'Whoops!' is displayed as greeting.

You can override this with the `greeting()` method:

```
public function toMail($notifiable)
{
    return (new MailMessage)
                ->greeting("Hi {$notifiable->name},")
                ->line('The introduction to the notification.')
                ->action('Notification Action', 'https://laravel.com')
                ->line('Thank you for using our application!');
}
```