README: fix license typo
README: fix license typo

The README contains a reference to GPL 2.1, which does not exist.
COPYING contains 2.0 so change the README to follow.

Signed-off-by: Jim Fehlig <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt