README: s/GPLv2 or older/GPLv2 or later/.

gitstats is dual-licensed under GPLv2/GPLv3, so naturally the contributions are
asked to be GPLv2+.

Thanks-to: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>

doc/README

 
 Contributions
 =============
-Patches should be sent under "GPLv2 or older" license - this will allow
+Patches should be sent under "GPLv2 or later" license - this will allow
 upgrading to newer GPL versions if they are sensible.