Fix outdated @flattenProperty link in no-openapi documentation

[Copilot]

The @flattenProperty decorator link in the no-openapi rule documentation points to a non-existent anchor. The decorator was moved to the Azure.ClientGenerator.Core.Legacy namespace but the link was not updated.

• Updated anchor from #@Azure.ClientGenerator.Core.flattenProperty to #@Azure.ClientGenerator.Core.Legacy.flattenProperty

Original prompt

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This section details on the original issue you should resolve

<issue_title>[doc] no-openapi document is out of date</issue_title>
<issue_description>The link of @flattenProperty is outdated in this document: https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-openapi/.</issue_description>

<agent_instructions>Change the link to @flattenProperty to the actual jsdoc link in Azure.Core.Legacy namespace</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [doc] no-openapi document is out of date #3575

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