blackbox: update README stratis_cert.py example block devices
blackbox: update README stratis_cert.py example block devices

The example execution of the stratis_cert.py test shows the block
devices "/dev/vda", "/dev/vdb", and "/dev/vdc" as the test devices.
However, there's a good chance that the test system has its operating
system installed on "/dev/vda", so change the test devices to
"/dev/vdb", "/dev/vdc", and "dev/vdd".

Signed-off-by: Bryan Gurney <94498243b15882748b18aec9e6dc02da713818c8@redhat.com>

stratis-storage/stratis-cli,stratis-storage/stratis-cli