Rename confusing _resource arg in InvitationsController

Given this method argument is not used, I think it's clearer not to give
it a name, particularly one which is easily confused with the
`#resource` accessor method.

app/controllers/invitations_controller.rb

@@ -59,7 +59,7 @@ def resend
 
 private
 
-  def after_invite_path_for(_resource)
+  def after_invite_path_for(_)
     if invitee_requires_2sv(resource)
       users_path
     else