Update README-windows.txt

python OVirtGuestService.py install should be without "-" near install to work as expected. Tested on 3.3. and windows 2003 and 2008

Change-Id: I0e5eb8a6a78a9a6c1a036a4fb451e75bb6fb2d3e
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>