vty: Fix documentation for 'no cs downgrade-threshold'

The NO_STR is missing, this commit adds it.

Fixes: Jenkins #603
Sponsored-by: On-Waves ehf
diff --git a/src/pcu_vty.c b/src/pcu_vty.c
index 05c4b7a..4c0730c 100644
--- a/src/pcu_vty.c
+++ b/src/pcu_vty.c
@@ -631,7 +631,7 @@
 DEFUN(cfg_pcu_no_cs_downgrade_thrsh,
       cfg_pcu_no_cs_downgrade_thrsh_cmd,
       "no cs downgrade-threshold",
-      CS_STR CS_DOWNGRADE_STR)
+      NO_STR CS_STR CS_DOWNGRADE_STR)
 {
 	struct gprs_rlcmac_bts *bts = bts_main_data();