[5.7] Correct Mail set locale code snippet

[derekmd]

Fixes: laravel/framework#27023

The documented mailable localization example:

Mail::to($request->user())->send(
    (new OrderShipped($order))->locale('es')
);

would always be sent using the default locale since PendingMail@fill() overrides 'es' with the PendingMail default value which is null.

Mail::to($request->user())->locale('es') should instead be used to set the locale through the mail service.

There may have been confusion since laravel/framework#25752 started allowing this call for a Notification:

$user->notify((new OrderConfirmation($order))->locale('en'))

since there is no intermediate object to assign a locale to.