[ONOS-2499]update accordingly for ONOS-2408's review recommendation

Change-Id: I24e7c4acbe38e4286b994c5bf50d22def945fb29
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/OvsdbMap.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/OvsdbMap.java
index beb9222..fce3f69 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/OvsdbMap.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/OvsdbMap.java
@@ -38,7 +38,7 @@
      * @param map java.util.Map
      */
     private OvsdbMap(Map map) {
-        checkNotNull(map, "map is not null");
+        checkNotNull(map, "map cannot be null");
         this.map = map;
     }