Add Photoprism Demo url (#3478)

README.md

@@ -1566,7 +1566,7 @@ A [gallery](https://en.wikipedia.org/wiki/Gallery_Software) is software that hel
 - [Mejiro](https://github.com/dmpop/mejiro) - An easy-to-use PHP web application for instant photo publishing. `GPL-3.0` `PHP`
 - [Photo Stream](https://github.com/waschinski/photo-stream) - Minimalist self-hosted photo stream. ([Demo](https://floremotion.de/)) `MIT` `Ruby`
 - [Photonix](https://photonix.org/) - A new web-based photo management application with object recognition, location awareness, color analysis and other ML algorithms. ([Demo](https://demo.photonix.org/), [Source Code](https://github.com/photonixapp/photonix)) `AGPL-3.0` `Python`
-- [PhotoPrism](https://photoprism.org) - Personal photo management powered by Go and Google TensorFlow.  Browse, organize, and share your personal photo collection, using the latest technologies to automatically tag and find pictures. ([Source Code](https://github.com/photoprism/photoprism)) `MIT` `Go`
+- [PhotoPrism](https://photoprism.org) - Personal photo management powered by Go and Google TensorFlow.  Browse, organize, and share your personal photo collection, using the latest technologies to automatically tag and find pictures. ([Demo](https://demo.photoprism.app/library/browse), [Source Code](https://github.com/photoprism/photoprism)) `MIT` `Go`
 - [Photoview](https://photoview.github.io/) - A simple and user-friendly Photo Gallery for personal servers. It is made for photographers and aims to provide an easy and fast way to navigate directories, with thousands of high resolution photos. ([Demo](https://photos.qpqp.dk/), [Source Code](https://github.com/photoview/photoview)) `GPL-3.0` `Go`
 - [PiGallery 2](https://bpatrik.github.io/pigallery2/) - A directory-first photo gallery website, with a rich UI, optimised for running on low resource servers. ([Source Code](https://github.com/bpatrik/pigallery2)) `MIT` `Docker/Nodejs`
 - [Piwigo](https://piwigo.org/) - Photo gallery software for the web, built by an active community of users and developers. ([Source Code](https://github.com/Piwigo/Piwigo)) `GPL-2.0` `PHP`