Update CITATION.cff

Author: jspaaks

CITATION.cff

@@ -28,5 +28,5 @@ license: "Apache-2.0"
 message: "If you use this software, please cite it using these metadata."
 repository-code: "https://github.com/xenon-middleware/xenon-tutorial"
 title: "xenon tutorial"
-version: "2.0.1"
+version: "3.0.4"
 ...