citations: Update year of envisioned NIFTy.re publication

docs/source/user/citations.rst

@@ -8,7 +8,7 @@ To cite the software library NIFTy, please use the following references:
       author       = {{Edenhofer}, Gordian and {Frank}, Philipp and {Leike}, Reimar H. and {Roth}, Jakob and {Guerdi}, Massin and {Enßlin}, Torsten A.},
       title        = {{Re-Envisioning Numerical Information Field Theory (NIFTy.re): A Library for Gaussian Processes and Variational Inference}},
       keywords     = {Software},
-      year         = 2023,
+      year         = 2024,
       howpublished = {in preparation}
     }
     @article{nifty5,