fix spelling detected by lintian

Change-Id: I01e54b5cf111677079a8ad57645d3ceb7834702a
diff --git a/include/osmocom/msc/signal.h b/include/osmocom/msc/signal.h
index 16b5678..c7f8991 100644
--- a/include/osmocom/msc/signal.h
+++ b/include/osmocom/msc/signal.h
@@ -89,7 +89,7 @@
 	struct vty *vty;
 };
 struct sms_signal_data {
-	/* The transaction where this occured */
+	/* The transaction where this occurred */
 	struct gsm_trans *trans;
 	/* Can be NULL for SMMA */
 	struct gsm_sms *sms;