Fix a typo in 'attach_volume' docstring
Fix a typo in 'attach_volume' docstring

Removed redundant 's'

Change-Id: I01b5132c86c64bb6396b515aace0f022315a9aff

cisco-openstack/tempest,cisco-openstack/tempest,Juniper/tempest,openstack/tempest,masayukig/tempest,vedujoshi/tempest,masayukig/tempest,Juniper/tempest,vedujoshi/tempest,openstack/tempest