[F#/iOS] Reference hint fix for Json.NET.

In case I'm not the only one seeing this...

In the F# iOS project, the Json.NET NuGet reference was causing a build error trying to find the restored DLL on a fresh copy of the repo. I just lined up the hint path with the current location where Xamarin Studio restores NuGet packages.

No fix needed for the F# Android project. That project uses a Xamarin Component version of Json.NET and [points to the proper location](https://github.com/xamarin/xamarin-store-app/blob/master/F%23/XamarinStore.Droid/XamarinStore.fsproj#L56).