fix doi for OPM reference

src/docs/JOSS/paper.bib

@@ -169,7 +169,7 @@ @article{RASMUSSEN2021159
 year = {2021},
 note = {Development and Application of Open-source Software for Problems with Numerical PDEs},
 issn = {0898-1221},
-doi = {https://doi.org/10.1016/j.camwa.2020.05.014},
+doi = {10.1016/j.camwa.2020.05.014},
 url = {https://www.sciencedirect.com/science/article/pii/S0898122120302182},
 author = {Atgeirr Flø Rasmussen and Tor Harald Sandve and Kai Bao and Andreas Lauser and Joakim Hove and Bård Skaflestad and Robert Klöfkorn and Markus Blatt and Alf Birger Rustad and Ove Sævareid and Knut-Andreas Lie and Andreas Thune},
 abstract = {The Open Porous Media (OPM) initiative is a community effort that encourages open innovation and reproducible research for simulation of porous media processes. OPM coordinates collaborative software development, maintains and distributes open-source software and open data sets, and seeks to ensure that these are available under a free license in a long-term perspective. In this paper, we present OPM Flow, which is a reservoir simulator developed for industrial use, as well as some of the individual components used to make OPM Flow. The descriptions apply to the 2019.10 release of OPM.}