Updates Readme

Small change in NSNotificationCenter -> NotificationCenter

README.md

@@ -210,7 +210,7 @@ and for stopping notifications
 
 ```swift
 reachability.stopNotifier()
-NSNotificationCenter.default.removeObserver(self,
+NotificationCenter.default.removeObserver(self,
                                                     name: ReachabilityChangedNotification,
                                                     object: reachability)
 ```