cli : glusterd getspec usage correction

gluster system getspec asks to input volume-id instead of volume name

Change-Id: I0baef9c937fc8948477aa65c8f98916b28c1e484
BUG: 1121870
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/8342
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>