Example uses deprecated value `MessageBoxIcon.Question`

The `MessageBoxIcon.Question` value is deprecated. The example should use a different icon, for example `MessageBoxIcon.Warning`.

See [`MessageBoxIcon.Question`](https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.messageboxicon?view=netframework-4.7.2#System_Windows_Forms_MessageBoxIcon_Question)

Note that the description text in front of the example references the "question mark".

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#### Document Details

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* ID: ffbc3a27-166c-b8d8-4471-bc9901897fe9
* Version Independent ID: bab01f6b-f648-f159-990b-39ecd1553f2b
* Content: [MessageBoxDefaultButton Enum (System.Windows.Forms)](https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.messageboxdefaultbutton?view=netframework-4.7.2)
* Content Source: [xml/System.Windows.Forms/MessageBoxDefaultButton.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Windows.Forms/MessageBoxDefaultButton.xml)
* Product: **.net**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnet-bot**