Update README.md

Co-authored-by: Sebastian Ehlert <28669218+awvwgk@users.noreply.github.com>

Author: sunqm

README.md

@@ -13,7 +13,7 @@ dftd3 package provides a drop-in replacement of pyscf.dftd3.itrf. For example
 
 ```
 from pyscf import gto
-import dftd3.pyscf as itrf
+import dftd3.pyscf as d3
 
 mol = gto.M(
     atom = ''' O    0.00000000    0.00000000   -0.11081188