Move zfs options doc to the storage section.

Zfs options belong to the storage section, but they were added under the exec driver section.

Signed-off-by: David Calavera <david.calavera@gmail.com>

docs/reference/commandline/cli.md

@@ -474,8 +474,6 @@ Currently supported options of `devicemapper`:
     > Otherwise, set this flag for migrating existing Docker daemons to
     > a daemon with a supported environment.
 
-### Docker execdriver option
-
 Currently supported options of `zfs`:
 
  * `zfs.fsname`
@@ -488,6 +486,8 @@ Currently supported options of `zfs`:
 
         $ docker -d -s zfs --storage-opt zfs.fsname=zroot/docker
 
+### Docker execdriver option
+
 The Docker daemon uses a specifically built `libcontainer` execution driver as
 its interface to the Linux kernel `namespaces`, `cgroups`, and `SELinux`.