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[bot]]

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

[learn-build-service-prod[bot]]

Learn Build status updates of commit cfe9ad0:

:white_check_mark: Validation status: passed

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

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel