use single hyphens in dataset card

README.md

@@ -15,8 +15,8 @@ size_categories:
 
 This dataset contains 181,898 nonequilibrium molecular geometries derived from
 QMugs molecules with 10-15 heavy atoms. These are the structures for which
-Kohn--Sham calculations converged and labels were generated. The 31,582 raw
-geometries with nonconverged Kohn--Sham calculations are not included.
+Kohn-Sham calculations converged and labels were generated. The 31,582 raw
+geometries with nonconverged Kohn-Sham calculations are not included.
 
 ## Generation