修复 Mysql 教程 update 失败的问题

[OnedayLiu]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

update 方法，如果自定义主键名称不是 id，以下写法会报错：

const row = {
  custom_id: 123,
  name: 'xx'
};
const result = await this.app.mysql.update('posts', row); 

[OnedayLiu]

ping @dead-horse

[codecov]

Codecov Report

Merging #2165 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2165   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          29       29           
  Lines         749      749           
=======================================
  Hits          746      746           
  Misses          3        3

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