services: wireguard: Shepherd 'stop' method really stops.

Until now the 'stop' method would return #t, which shepherd would
interpret as "services could not be stopped".

* gnu/services/vpn.scm (wireguard-shepherd-service): Have 'stop' return #f.

gnu/services/vpn.scm

@@ -814,7 +814,8 @@ PostUp = ~a set %i private-key ~a
              (start #~(lambda _
                        (invoke #$wg-quick "up" #$config)))
              (stop #~(lambda _
-                       (invoke #$wg-quick "down" #$config)))
+                       (invoke #$wg-quick "down" #$config)
+                       #f))                       ;stopped!
              (documentation "Run the Wireguard VPN tunnel"))))))
 
 (define wireguard-service-type