Update to F# 6

I updated the example to the current standards. 

https://devblogs.microsoft.com/dotnet/whats-new-in-fsharp-6/#bringing-f-forward-reducing-use-of-rarely-used-symbolic-operators

public/samples/games/undertone.fs

@@ -236,8 +236,8 @@ module NoteSequencer =
         if wanted = 0 then Seq.empty else
         seq { use e = source.GetEnumerator()
               let count = ref 0
-              while e.MoveNext() && !count < wanted do
-                incr count
+              while e.MoveNext() && count.Value < wanted do
+                count.Value <- count.Value + 1 
                 yield e.Current }
 
     // function that does a function the describes how a note should be played and list of chords
@@ -382,4 +382,4 @@ let baaBaaBlackSheepChorus =
           yield! makeNote crotchet Note.D
           yield! makeNote crotchet Note.C }
 
-Html.loadSound baaBaaBlackSheepChorus
+Html.loadSound baaBaaBlackSheepChorus