man: fixed typo in "volume statedump" entry of manpage

Patch to simply correct a spelling mistake as reported under: "894355 - spelling mistake?"

Change-Id: I2a93b3fc34a8e39e6fe775f3747d91c6fb489f43
Signed-off-by: xarlos <xarlos@xarlos.me>
Reviewed-on: http://review.gluster.org/4379
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>