Fix a type in SNMP example that may lead to confusion.

The number in SNMP_CREATE_TREE_NODE(0, private_nodes); is not actually used, but for consistency, it should be the same as in the base OID declaration.

examples/snmp/snmp_private_mib/lwip_prvmib.c

@@ -160,7 +160,7 @@ static const struct snmp_tree_node enterprises_node = SNMP_CREATE_TREE_NODE(1, e
 static const struct snmp_node* const private_nodes[] = {
   &enterprises_node.node
 };
-static const struct snmp_tree_node private_root = SNMP_CREATE_TREE_NODE(0, private_nodes);
+static const struct snmp_tree_node private_root = SNMP_CREATE_TREE_NODE(4, private_nodes);
 
 static const u32_t prvmib_base_oid[] = { 1,3,6,1,4 };
 const struct snmp_mib mib_private = SNMP_MIB_CREATE(prvmib_base_oid, &private_root.node);