smpp: Fix help string in vty for "alert-notifications"

Without the 'no' prefix, this command is about enabling, not disabling

Change-Id: I3019a0d9271e73030b9d7cd555761e17358ec812
diff --git a/src/libmsc/smpp_vty.c b/src/libmsc/smpp_vty.c
index 9026f6c..31e2090 100644
--- a/src/libmsc/smpp_vty.c
+++ b/src/libmsc/smpp_vty.c
@@ -501,7 +501,7 @@
 
 DEFUN(cfg_esme_alert_notif, cfg_esme_alert_notif_cmd,
 	"alert-notifications",
-	"Disable sending of SMPP Alert Notifications for this ESME")
+	"Enable sending of SMPP Alert Notifications for this ESME")
 {
 	struct osmo_smpp_acl *acl = vty->index;