Fix typo in PingPong example (#767)

examples/pingpong/main.go

@@ -51,7 +51,7 @@ func main() {
 }
 
 // This function will be called (due to AddHandler above) every time a new
-// message is created on any channel that the autenticated bot has access to.
+// message is created on any channel that the authenticated bot has access to.
 func messageCreate(s *discordgo.Session, m *discordgo.MessageCreate) {
 
 	// Ignore all messages created by the bot itself