doc: Fix "desktop" OS config example.

* gnu/system/examples/desktop.tmpl (file-systems): Add
'file-system-label' call in 'device' field.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>

gnu/system/examples/desktop.tmpl

@@ -26,7 +26,7 @@
           (type luks-device-mapping))))
 
   (file-systems (cons (file-system
-                        (device "my-root")
+                        (device (file-system-label "my-root"))
                         (mount-point "/")
                         (type "ext4")
                         (dependencies mapped-devices))