[marytts] Fix broken link (#21437)

Signed-off-by: Leo Siepel <leosiepel@gmail.com>

Author: lsiepel

bundles/org.openhab.voice.marytts/README.md

@@ -1,6 +1,6 @@
 # Mary Text-to-Speech
 
-The Mary Text-to-Speech (TTS) service is a pure Java implementation of a TTS service, which uses the [MaryTTS](http://mary.dfki.de/) project of DFKI.
+The Mary Text-to-Speech (TTS) service is a pure Java implementation of a TTS service, which uses the [MaryTTS](https://mary.opendfki.de/) project of DFKI.
 
 While it provides good quality results, it must be noted that it is too heavy-weight for most embedded hardware like a Raspberry Pi. When using this service, you should be running openHAB on some real server instead.