minor logs changes at http for network update openflow rules

Change-Id: Ifce2878f0c71a92ab0fa6d165792d170e7ae553d
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>
diff --git a/ovim.py b/ovim.py
index 15bde75..6e8f8da 100644
--- a/ovim.py
+++ b/ovim.py
@@ -939,7 +939,7 @@
                     r, c = ofc_t[switch_dpid].insert_task("update-net", net_id)
 
         if r < 0:
-            message = "Cannot insert a task for updating network '$s', %s", net_id, c
+            message = "Cannot insert a task for updating network '{}', {}".format(net_id, c)
             self.logger.error(message)
             raise ovimException(message, HTTP_Internal_Server_Error)