docs: added kube-system namespace to the registry sample's pvc in cephfs
docs: added kube-system namespace to the registry sample's pvc in cephfs

The CephFS documentation page's kube-registry.yaml PVC definition was missing the namespace property.
The YAML shown in the docs now reflects the file in cluster/examples/kubernetes/ceph/csi/cephfs/kube-registry.yaml

Signed-off-by: Léo Schoukroun <746b7133b662c176121ef7645747f27ca7c36cda@yahoo.fr>

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