[Modify] Polish it

websocket-sharp/WebSocket.cs

@@ -794,22 +794,22 @@ namespace WebSocketSharp
       message = null;
 
       if (!connecting && _readyState == WebSocketState.Connecting) {
-        message = "This operation isn't available in: connecting";
+        message = "This operation is not available in: connecting";
         return false;
       }
 
       if (!open && _readyState == WebSocketState.Open) {
-        message = "This operation isn't available in: open";
+        message = "This operation is not available in: open";
         return false;
       }
 
       if (!closing && _readyState == WebSocketState.Closing) {
-        message = "This operation isn't available in: closing";
+        message = "This operation is not available in: closing";
         return false;
       }
 
       if (!closed && _readyState == WebSocketState.Closed) {
-        message = "This operation isn't available in: closed";
+        message = "This operation is not available in: closed";
         return false;
       }