Remove confusing exception variable name

[gewarren]

Fixes #10552.

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

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Learn Build status updates of commit 0fabf3c:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System/AggregateException/Overview/exception1.cs	:white_check_mark:Succeeded	View

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