tests/example-gatt-server: Fix using GATT_CHRC_IFACE in Descriptor

This patch changes GATT_CHRC_IFACE to GATT_DESC_IFACE when querying
properties for Descriptor class.
diff --git a/test/example-gatt-server b/test/example-gatt-server
index 44ceb70..bac1687 100755
--- a/test/example-gatt-server
+++ b/test/example-gatt-server
@@ -234,7 +234,7 @@
         if interface != GATT_DESC_IFACE:
             raise InvalidArgsException()
 
-        return self.get_properties()[GATT_CHRC_IFACE]
+        return self.get_properties()[GATT_DESC_IFACE]
 
     @dbus.service.method(GATT_DESC_IFACE,
                         in_signature='a{sv}',