man: Ceph is also an object store

Replace

   Ceph distributed file system

with

   Ceph distributed storage system

to help reduce the idea that Ceph is just a file system.

Signed-off-by: Loic Dachary <loic@dachary.org>
man: update man/ from doc/man/8

As explained in admin/manpage-howto.txt

Signed-off-by: Loic Dachary <loic@dachary.org>