updates zenodo badge for SPECFEM2D v8.1.0

Author: danielpeter

README.md

@@ -1,6 +1,6 @@
 # SPECFEM2D
 
-[![DOI](https://zenodo.org/badge/14293189.svg)](https://zenodo.org/badge/latestdoi/14293189)<br>
+[![DOI](https://zenodo.org/badge/14293189.svg)](https://zenodo.org/doi/10.5281/zenodo.7434515)
 
 SPECFEM2D allows users to perform 2D and 2.5D (i.e., axisymmetric) simulations
 of acoustic, elastic, viscoelastic, and poroelastic seismic wave propagation.