Fix use of CryptoStream examples

[vcsjones]

Summary

The example code read from the backing MemoryStream before the CryptoStream flushed the final block to the underlying stream.

Fixes #10119

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones

[learn-build-service-prod]

Learn Build status updates of commit cfe9ad0:

✅ Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System.Security.Cryptography/Aes/Overview/program.cs	✅Succeeded	View
snippets/csharp/System.Security.Cryptography/AesCryptoServiceProvider/Overview/program.cs	✅Succeeded	View
snippets/csharp/System.Security.Cryptography/AesManaged/Overview/program.cs	✅Succeeded	View
snippets/csharp/System.Security.Cryptography/RijndaelManaged/Overview/class1.cs	✅Succeeded	View

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