itemsInBags will be different from 500 (dotnet#8043)

* itemsInBags will be different 500

itemsInBags will be different 500 without atomic operation.

* Add System.Threading package

Add using System.Threading for function used.

Author: chenkaigithub

snippets/csharp/System.Collections.Concurrent/ConcurrentBag/concbag.cs

@@ -3,6 +3,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using System.Threading;
 
 class ConcurrentBagDemo
 {
@@ -36,7 +37,7 @@ static void Main()
                 if (cb.TryTake(out item))
                 {
                     Console.WriteLine(item);
-                    itemsInBag++;
+                    Interlocked.Increment(ref itemsInBag);
                 }
             }));
         }