iprule: Align help text with man page synopsis

The help text was misleading: One could think it is possible to list
rules by selector, which would be nice but isn't. This change also
clarifies that 'ip rule' defaults to 'list' if no further arguments are
given.
Signed-off-by: Phil Sutter <phil@nwl.cc>

ip/iprule.c

@@ -33,8 +33,9 @@ static void usage(void) __attribute__((noreturn));
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: ip rule [ list | add | del | flush | save ] SELECTOR ACTION\n");
-	fprintf(stderr, "       ip rule restore\n");
+	fprintf(stderr, "Usage: ip rule { add | del } SELECTOR ACTION\n");
+	fprintf(stderr, "       ip rule { flush | save | restore }\n");
+	fprintf(stderr, "       ip rule [ list ]\n");
 	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");