add missing await

Ix.NET/Source/System.Interactive.Async/Concatenate.cs

@@ -223,7 +223,7 @@ namespace System.Linq
 
                     checked
                     {
-                        count += await source.Count(cancellationToken);
+                        count += await source.Count(cancellationToken).ConfigureAwait(false);
                     }
                 }