"Implementing the GetInstance Method" on ICustomMarshaler needs language-specific example

[dlech]

This sample code snippet currently uses C++ for all languages (C#, VB), so copy and paste does not work.

https://github.com/dotnet/docs/blob/master/xml/System.Runtime.InteropServices/ICustomMarshaler.xml#L117

[carlossanlop]

Tagging area owners @AaronRobinsonMSFT @jkoritzinsky for triaging.