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: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

README.md

@@ -30,7 +30,7 @@ License
 The libvirt C API is distributed under the terms of GNU Lesser General
 Public License, version 2.1 (or later). Some parts of the code that are
 not part of the C library may have the more restrictive GNU General
-Public License, version 2.1 (or later). See the files `COPYING.LESSER`
+Public License, version 2.0 (or later). See the files `COPYING.LESSER`
 and `COPYING` for full license terms & conditions.