fix(telegram): passing ParseMode when sending message (#4027)

Before commit [1], the message parse mode value is the same
as bot parse mode. Therefore, we don't need to pass ParseMode
whenever sending message. But after that commit, if we don't
pass SendOpts's ParseMode, the library will use the default
mode which is empty string "".

[1] 864bef4e4d

Signed-off-by: Kien Nguyen <kiennt2609@gmail.com>

notify/telegram/telegram.go

@@ -94,6 +94,7 @@ func (n *Notifier) Notify(ctx context.Context, alert ...*types.Alert) (bool, err
 		DisableNotification:   n.conf.DisableNotifications,
 		DisableWebPagePreview: true,
 		ThreadID:              n.conf.MessageThreadID,
+		ParseMode:             n.conf.ParseMode,
 	})
 	if err != nil {
 		return true, err