iprule: add missing nat keyword to help text

Signed-off-by: Phil Sutter <phil@nwl.cc>

ip/iprule.c

@@ -39,6 +39,7 @@ static void usage(void)
 	fprintf(stderr, "SELECTOR := [ not ] [ from PREFIX ] [ to PREFIX ] [ tos TOS ] [ fwmark FWMARK[/MASK] ]\n");
 	fprintf(stderr, "            [ iif STRING ] [ oif STRING ] [ pref NUMBER ]\n");
 	fprintf(stderr, "ACTION := [ table TABLE_ID ]\n");
+	fprintf(stderr, "          [ nat ADDRESS ]\n");
 	fprintf(stderr, "          [ realms [SRCREALM/]DSTREALM ]\n");
 	fprintf(stderr, "          [ goto NUMBER ]\n");
 	fprintf(stderr, "          SUPPRESSOR\n");