修复文档中sink.label-prefix 的默认值说明

[aubdiy]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Problem Summary(Required) ：

sink.label-prefix 配置默认值描述错误, 代码中是 'flink' 字符串

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function

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