Update link to F# Quick Guides: Object Oriented Programming

Microsoft have moved their blogs around.  The link needs to be updated to https://docs.microsoft.com/en-gb/archive/blogs/timng/f-quick-guides-object-oriented-programming. 

I've also made a copy of it in a [gist](https://gist.github.com/davidglassborow/7f40c016bf9dbd2b230fdb0f8a478ef5) in case they remove it forever.