fix NIFTy 5 ASCL citation

docs/source/citations.rst

@@ -2,8 +2,8 @@ NIFTy-related publications
 ==========================
 
 ::
-    @article{asclnifty6,
-      title={NIFTy6: Numerical Information Field Theory v5},
+    @article{asclnifty5,
+      title={NIFTy5: Numerical Information Field Theory v5},
       author={Arras, Philipp and Baltac, Mihai and Ensslin, Torsten A and Frank, Philipp and Hutschenreuter, Sebastian and Knollmueller, Jakob and Leike, Reimar and Newrzella, Max-Niklas and Platz, Lukas and Reinecke, Martin and others},
       journal={Astrophysics Source Code Library},
       year={2019}