doc/install/debian.rst: fix typo in link ref; broke doc build

Signed-off-by: Dan Mick <dan.mick@inktank.com>
rbd: allow copy of zero-length images.  Includes simple test.

Fixes: #3765
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge git://github.com/ceph/ceph