remove dead link to Open Event Server demo

README.md

@@ -1164,7 +1164,7 @@ See also _[Monitoring](#monitoring)_
 - [Kyélà](http://kyela.net/) - Participation polls for group events. ([Demo](https://kyela.net/concert/), [Source Code](https://github.com/abienvenu/Kyela)) `AGPL-3.0` `PHP`
 - [LimeSurvey](https://www.limesurvey.org) - Feature-rich Open Source web based polling software. Supports extensive survey logic. ([Demo](https://demo.limesurvey.org), [Source code](https://github.com/LimeSurvey/LimeSurvey)) `GPL-2.0` `PHP`
 - [Nuages](https://nuages.domainepublic.net/) - Collaborative meeting poll system, similar to doodle or rdvz. ([Source Code](http://git.domainepublic.net/?p=nuages.git;a=tree)) `GPL-3.0` `Python`
-- [Open Event Server](https://github.com/fossasia/open-event-server) - Enables organizers to manage events from concerts to conferences and meet-ups. ([Demo](https://open-event-api.herokuapp.com/)) `GPL-3.0` `Python`
+- [Open Event Server](https://github.com/fossasia/open-event-server) - Enables organizers to manage events from concerts to conferences and meet-ups. `GPL-3.0` `Python`
 - [PHPBack](http://www.phpback.org) - The open source feedback system. ([Demo](http://www.phpback.org/demo/), [Source Code](https://github.com/ivandiazwm/phpback)) `GPL-3.0` `PHP`
 
 ### Booking and Scheduling