Update Program.cs sample code to use the Using block for IDisposable objects.

[ssdatye]

Updated to correctly reflect the following:

1. Using block for unmanaged object that implements IDisposable
2. Removed explict call to Dispose()
3. Added explicit call to Save() methods of the worksheet and workbook

[learn-build-service-prod]

Learn Build status updates of commit 7ddd960:

✅ Validation status: passed

File	Status	Preview URL	Details
samples/spreadsheet/working_with_sheets/cs/Program.cs	✅Succeeded

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