Update Framework-Structure.md

[Rick-van-Dam]

Summary

This article is still mentioning PCL and netframework 4.5. Update this to netstandard 2.0 and netframework 4.7.2 (or maybe just don't mention the versions anymore?)

@ilexp do you still have that diagram somewhere so we can edit it easily or do we have to create a new one?

[ilexp]

It's a Google Docs drawing. I've put it in a folder that I'd share with you @Barsonax and @SirePi - can you send me your Google account e-mail addresses via PN in the chat?