Update push-pull.md

sink is pull-socket, not push socket

Author: moreal

docs/push-pull.md

@@ -37,7 +37,7 @@ Here is what we are trying to achieve :
                 Console.WriteLine("====== VENTILATOR ======");
 
                 using (var sender = new PushSocket("@tcp://*:5557"))
-                using (var sink = new PushSocket(">tcp://localhost:5558"))
+                using (var sink = new PullSocket(">tcp://localhost:5558"))
                 {
                     Console.WriteLine("Press enter when worker are ready");
                     Console.ReadLine();