readme: contact: matrix to matrix.to, server -> channel

README.md

@@ -145,5 +145,5 @@ lines). If you're running the Docker image, you can do this:
 ## Contact
 
 Feel free to join our Freenode IRC channel at #nitter, or our
-[Matrix server](https://riot.im/app/#/room/#nitter:matrix.org).
+[Matrix channel](https://matrix.to/#/#nitter:matrix.org).
 You can email me at zedeus@pm.me if you wish to contact me personally.