Update External Link Reference in trace-span-spec-reference.md

[liucheng-ms]

Description

This pull request updates an external link reference within a trace-span-spec-reference.md to ensure accuracy and relevance of the documentation. The modification corrects the hyperlink to point to the intended web resource.

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• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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