ceph: remove Sage's git tree from documentation

Sage's git tree has not been pushed to in years, and it was removed in
commit 3a5ccecd

 ("MAINTAINERS: remove myself as ceph co-maintainer"),
so it is better to remove it in the documentation too.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>