bib: fix fieldstone

fixes #6183

Author: tjhei

benchmarks/viscosity_grooves/doc/viscosity_grooves.md

@@ -22,7 +22,7 @@ where $\epsilon$ controls the viscosity contrast. It is easy to verify that
 the flow is incompressible as the velocity field satisfies
 $\nabla\cdot \mathbf u = 0$. The right hand side term of the Stokes equation
 is obtained by inserting the expressions for velocity, pressure and viscosity
-in the momentum conservation equation, see {cite:t}`thieulot:2019` for details. The
+in the momentum conservation equation, see {cite:t}`fieldstone` for details. The
 velocity, pressure and right hand side magnitude are shown in {numref}`fig:benchmark-grooves-3x3`
 for $L=3$ and $\epsilon=0.1$.
 

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}
+@misc{fieldstone,
+author = "Cedric Thieulot",
+title = "{Fieldstone: a computational geodynamics (self-)teaching tool}",
+year = "2023",
+url = {https://cedrict.github.io/}
 }
 
 @incollection{john:knobloch:2006,