[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/schema/DatabaseSchema.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/DatabaseSchema.java
index 3c29139..298afc6 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/DatabaseSchema.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/DatabaseSchema.java
@@ -40,9 +40,9 @@
      */
     public DatabaseSchema(String name, String version,
                           Map<String, TableSchema> tableSchemas) {
-        checkNotNull(name, "name is not null");
-        checkNotNull(version, "version is not null");
-        checkNotNull(tableSchemas, "tableSchemas is not null");
+        checkNotNull(name, "name cannot be null");
+        checkNotNull(version, "version cannot be null");
+        checkNotNull(tableSchemas, "tableSchemas cannot be null");
         this.name = name;
         this.version = version;
         this.tableSchemas = tableSchemas;