remove zentao demo link (certificate expired)

- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/2266

README.md

@@ -1892,7 +1892,7 @@ _See also: [awesome-sysadmin/Code Review](https://github.com/awesome-foss/awesom
 - [Traq](https://traq.io/) - Project management and issue tracking system written in PHP. ([Source Code](https://github.com/nirix/traq)) `GPL-3.0` `PHP/Nodejs`
 - [Tuleap](https://www.tuleap.org/) - Tuleap is a libre suite to plan, track, code and collaborate on software projects. ([Source Code](https://tuleap.net/plugins/git/tuleap/tuleap/stable?p=tuleap%2Fstable.git&a=tree)) `GPL-2.0` `PHP`
 - [UVDesk](https://www.uvdesk.com/) - UVDesk community is a service oriented, event driven extensible opensource helpdesk system that can be used by your organization to provide efficient support to your clients effortlessly whichever way you imagine. ([Demo](https://demo.uvdesk.com/), [Source Code](https://github.com/uvdesk/community-skeleton)) `MIT` `PHP`
-- [ZenTao](https://www.zentao.pm/) - An agile(scrum) project management system/tool. ([Demo](https://demo.zentao.pm/user-login.html), [Source Code](https://github.com/easysoft/zentaopms)) `ZPL-1.2` `PHP`
+- [ZenTao](https://www.zentao.pm/) - An agile(scrum) project management system/tool. ([Source Code](https://github.com/easysoft/zentaopms)) `ZPL-1.2` `PHP`
 
 
 ### Software Development - Testing