bib: fix fieldstone

fixes #6183

Author: tjhei

doc/sphinx/references.bib

@@ -11855,11 +11855,11 @@ @article{garzanti:etal:2018
 doi = {10.1016/j.earscirev.2017.11.012},
 }
 
-@Misc{thieulot:2019,
-  author = {C. Thieulot},
-  title  = {The Finite Element Method in Geodynamics},
-  url    = {https://github.com/cedrict/fieldstone/blob/master/manual.pdf},
-  year   = {2019}
+@article{fieldstone,
+author = "Cedric Thieulot",
+title = "{Fieldstone: a computational geodynamics (self-)teaching tool}",
+year = "2023",
+doi = "10.5194/egusphere-egu23-14212"
 }
 
 @incollection{john:knobloch:2006,