[ISSUE #1652] Update service description error in Open API Guide

[finalcola]

issue #1652 (comment)

What is the purpose of the change

set default value for protectThreshold to 0, as same as Open API Guide: https://nacos.io/en-us/docs/open-api.html#Update

Brief changelog

set default value for protectThreshold to 0

Verifying this change

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