Fix docs of MemoryExtensions.AsSpan<T>(this T[]? array, int start, int length)

[halgab]

Summary

I've spotted what looks like a bad copy/paste from the corresponding string extension method for method MemoryExtensions.AsSpan<T>(this T[]? array, int start, int length)

[learn-build-service-prod]

Learn Build status updates of commit 4c4c73f:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/MemoryExtensions.xml	✅Succeeded	View

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[BillWagner]

Thanks @halgab

I've reviewed this, and it LGTM. I'll now.