Update shield from gitter to matrix

Author: shiltemann

README.md

@@ -1,7 +1,8 @@
 <img src="./shared/images/GTNLogo1000.png" alt="GTN logo, a star on a coloured circle with the text Galaxy Training Network at an upward angle." width="400"  align="right" />
 
 # Galaxy Training Material 
-[![Deploy Website](https://github.com/galaxyproject/training-material/actions/workflows/deploy.yml/badge.svg)](https://github.com/galaxyproject/training-material/actions/workflows/deploy.yml) [![Gitter](https://badges.gitter.im/Galaxy-Training-Network/training-material.svg)](https://gitter.im/Galaxy-Training-Network/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Deploy Website](https://github.com/galaxyproject/training-material/actions/workflows/deploy.yml/badge.svg)](https://github.com/galaxyproject/training-material/actions/workflows/deploy.yml) ![Matrix](https://img.shields.io/matrix/gtn:matrix.org)
+
 
 This repository collects tutorials, slides, and exercises developed and maintained by the Galaxy community.