Use 'default.target' in systemd unit file example

Previously, 'local.target' was used which is not going to work
out-of-the-box for lots of users.
The 'default.target' is the default unit systemd starts at bootup and
hence what most users would be expected to use.

Signed-off-by: Tadej Janež <tadej.j@nez.si>

docs/admin/host_integration.md

@@ -74,7 +74,7 @@ a new service that will be started after the docker daemon service has started.
     ExecStop=/usr/bin/docker stop -t 2 redis_server
 
     [Install]
-    WantedBy=local.target
+    WantedBy=default.target
 
 If you need to pass options to the redis container (such as `--env`),
 then you'll need to use `docker run` rather than `docker start`. This will