Update QQ mail in settings.md

[QYunChao]

本人在配置邮箱通知的时候，发现 QQ 邮箱配置还需要主动开启 POP3/SMTP 服务，所以更新一下文档，以免后来人重走弯路

None

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[ruibaby]

需要同时修改 docs/user-guide/settings.md