Fix ref struct documentation link for Span<T>

[arphox]

Summary

Ref struct documentation probably has been moved, this PR aims to fix the wrong links I have found.

[learn-build-service-prod]

Learn Build status updates of commit aedf732:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Span`1.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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[BillWagner]

Thanks for catching these and fixing them @arphox

I've approved the changes, and I'll now.

You should see the changes on our live site in the next day or two, on our regular publishing cycle.