Fix admin search for VPN

coin/vpn/admin.py

@@ -23,9 +23,9 @@ class VPNConfigurationAdmin(ConfigurationAdminFormMixin, PolymorphicChildModelAd
     list_filter = ('activated',)
     search_fields = ('login', 'comment',
                      # TODO: searching on member directly doesn't work
-                     'administrative_subscription__member__first_name',
-                     'administrative_subscription__member__last_name',
-                     'administrative_subscription__member__email')
+                     'offersubscription__member__first_name',
+                     'offersubscription__member__last_name',
+                     'offersubscription__member__email')
     actions = (delete_selected, "generate_endpoints", "generate_endpoints_v4",
                "generate_endpoints_v6", "activate", "deactivate")
     exclude = ("password",)