docs: remove obsolete reference to v1alpha2 in design doc
docs: remove obsolete reference to v1alpha2 in design doc

The storageClassDeviceSets unnecessarily references the obsolete v1alpha2
version.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

rook/rook,rook/rook,rook/rook,rook/nfs,rook/rook,rook/nfs,rook/nfs,rook/rook