Fix inconsistent test case number in comments and console logs

[depth233]

The node (dog) was removed in test 12, not in test 11.

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Tagging subscribers to this area: @dotnet/area-system-collections

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Learn Build status updates of commit ec57a44:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System.Collections.Generic/LinkedListT/Overview/source.cs	:white_check_mark:Succeeded	View

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