Typo fix

Just a small typo fix.

conn.go

@@ -76,7 +76,7 @@ const (
 	// is UTF-8 encoded text.
 	PingMessage = 9
 
-	// PongMessage denotes a ping control message. The optional message payload
+	// PongMessage denotes a pong control message. The optional message payload
 	// is UTF-8 encoded text.
 	PongMessage = 10
 )