[doc] Fix Ceph sources

Ussuri edition

Change-Id: I558dea5d58997d415ed935fe0afd265701a552ab

Author: yoctozepto

doc/source/ceph_versions.csv

@@ -1,4 +1,4 @@
 Distro,Ceph source,Release
 CentOS,CentOS Storage SIG,Nautilus
 Ubuntu,Ubuntu Cloud Archive,Octopus
-Debian,Debian,Nautilus
+Debian,Debian Backports,Nautilus