Change Bitpoll Demo URL (#3388)

Changes the URL of the Bitpoll Demo to the official public instance, where third party users are able to register.

README.md

@@ -1572,7 +1572,7 @@ Software for organising [polls](https://en.wikipedia.org/wiki/Opinion_poll) and
 
 _Related: [Booking and Scheduling](#booking-and-scheduling)_
 
-- [Bitpoll](https://github.com/fsinfuhh/Bitpoll) - A web application for scheduling meetings and general polling. ([Demo](https://bitpoll.mafiasi.de/)) `GPL-3.0` `Python`
+- [Bitpoll](https://github.com/fsinfuhh/Bitpoll) - A web application for scheduling meetings and general polling. ([Demo](https://bitpoll.de/)) `GPL-3.0` `Python`
 - [Calagator](https://calagator.org/) - Event aggregator. ([Source Code](https://github.com/calagator/calagator)) `MIT` `Ruby`
 - [Christmas Community](https://github.com/Wingysam/Christmas-Community) - Create a simple place for your entire family to use to find gifts that people want, and to avoid double-gifting. `AGPL-3.0` `Docker/Nodejs`
 - [Claper](https://claper.co/) - The ultimate tool to interact with your audience. An open-source alternative to Slido, AhaSlides and Mentimeter. ([Source Code](https://github.com/ClaperCo/Claper)) `GPL-3.0` `Elixir/Docker`