up readme

Author: ejmeitz

README.md

@@ -20,9 +20,9 @@ fcc_crystal = FCC(a, :C, SVector(4,4,4))
 atoms = fcc_crystal.atoms
 to_xyz(fcc_crystal, raw"./positions_fcc.xyz")
 
-#Equivalently if you do not want to specify an atomic species
+#Equivalently if you do not want to specify an atomic mass
 a = 5.4u"Å"
-fcc_crystal = FCC(a, 12.01u"g/mol", SVector(4,4,4))
+fcc_crystal = FCC(a, 12.01u"g/mol", SVector(4,4,4); atomic_symbol = :C)
 atoms = fcc_crystal.atoms
 to_xyz(fcc_crystal, raw"./positions_fcc.xyz")
 ```