Update CITATION.cff

Co-authored-by: Peter Kalverla <peter.kalverla@gmx.com>

Author: SarahAlidoost

CITATION.cff

@@ -75,7 +75,7 @@ license: "CC-BY-4.0"
 message: "If you use this tutorial, please cite it using these metadata."
 repository-code: "https://github.com/ESMValGroup/ESMValTool_Tutorial/"
 title: "ESMValTool Tutorial"
-keyword:
+keywords:
   - Earth System Model Validation
   - Climate Science
 version: "v1.0.0"