remove problematic nullboard demo link

SSL_connect returned=1 errno=0 state=error: certificate verify failed

README.md

@@ -1486,7 +1486,7 @@ _See also [Project Management](#project-management) and [Ticketing](#ticketing).
 - [Kanbana](https://github.com/SrGMC/kanbana) - Create boards to track users and projects from flat markdown files. Forked from Crepido. `MIT` `Nodejs`
 - [Kanboard](http://kanboard.net/) - Simple and open source visual task board. ([Source Code](https://github.com/kanboard/kanboard)) `MIT` `PHP`
 - [myTinyTodo](http://www.mytinytodo.net/) - Simple way to manage your todo list in AJAX style. Uses PHP, jQuery, SQLite/MySQL. GTD compliant. ([Demo](http://www.mytinytodo.net/demo/), [Source Code](https://github.com/maxpozdeev/mytinytodo/)) `GPL-2.0` `PHP`
-- [Nullboard](https://github.com/apankrat/nullboard) - Single-page minimalist kanban board; compact, highly readable and quick to use. ([Demo](https://nullboard.io/preview)) `BSD-2-Clause` `Javascript`
+- [Nullboard](https://github.com/apankrat/nullboard) - Single-page minimalist kanban board; compact, highly readable and quick to use. `BSD-2-Clause` `Javascript`
 - [PHP Task/Todo list](https://github.com/lgg/tasks.php) - Simple task/todo list that uses a JSON text file for the tasks. `MIT` `PHP`
 - [Planka](https://planka.app/) - Open source Trello alternative. ([Demo](https://plankanban.github.io/planka/#/), [Source Code](https://github.com/plankanban/planka)) `MIT` `Nodejs`
 - [Restyaboard](http://restya.com/board/) - Open source Trello-like kanban board. ([Demo](http://restya.com/board/demo.html), [Source Code](https://github.com/RestyaPlatform/board)) `OSL-3.0` `PHP`