nfs: 'gluster volume help' should show the correct path for nfs.mount-rmtab

This has been fixed in newer releases with a more invasive change:
- build: make GLUSTERD_WORKDIR rely on localstatedir
  http://review.gluster.org/8246

For this release, it is sufficient to only correct the help text.

Change-Id: Id5db126e3b5f8b98c2810d5b5a6f7079200df612
BUG: 1147243
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/8874
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>