add new citation to docs

Author: MuellerSeb

docs/source/index.rst

@@ -124,7 +124,7 @@ Citation
 
 If you are using GSTools in your publication please cite our paper:
 
-| *Müller, S., Schüler, L., Zech, A., and Heße, F.: GSTools v1.3: A toolbox for geostatistical modelling in Python, Geosci. Model Dev. Discuss. [preprint], https://doi.org/10.5194/gmd-2021-301, in review, 2021.*
+| *Müller, S., Schüler, L., Zech, A., and Heße, F.: GSTools v1.3: a toolbox for geostatistical modelling in Python, Geosci. Model Dev., 15, 3161–3182, https://doi.org/10.5194/gmd-15-3161-2022, 2022.*
 
 You can cite the Zenodo code publication of GSTools by: