Update mongodbwriter.md

[wangxiaoming]

参数异常 测试不可用，将 "upsertInfo": { "isUpsert": "true", "upsertKey": "name" } 修改成 "writeMode": { "isReplace": "true", "replaceKey": "unique_id" }

才可以，参考了源码， 测试可用

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