nat: Fix the documentation issue with the osmux command

Documentation error (missing docs):
<command id='osmux (on|off)'>
        <param name='off' doc='(null)' />
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
index d5903d8..ab89db8 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
@@ -1177,7 +1177,7 @@
 	return CMD_SUCCESS;
 }
 
-#define OSMUX_STR "RTP multiplexing"
+#define OSMUX_STR "RTP multiplexing\n"
 DEFUN(cfg_bsc_osmux,
       cfg_bsc_osmux_cmd,
       "osmux (on|off)",