readme: replaced cdi clone url (#426)
readme: replaced cdi clone url (#426)

Download CDI section:
Replaced dead url (kubevirt.io/containerized-data-importer.git) with the github repo.

Change-Id: I122feda13f67a72ab573ebd0a412e92314904268
Signed-off-by: Daniel Erez <2f9e6efa5394039257912622b475da08d58ee4c7@redhat.com>
kubevirt/containerized-data-importer,kubevirt/containerized-data-importer,kubevirt/containerized-data-importer