ipaddrlabel: Improve help text precision

Neither 'list' nor 'flush' actions accept parameters, and with given
prefix the action keyword is not optional anymore.
Signed-off-by: Phil Sutter <phil@nwl.cc>

ip/ipaddrlabel.c

@@ -49,7 +49,8 @@ static void usage(void) __attribute__((noreturn));
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: ip addrlabel [ list | add | del | flush ] prefix PREFIX [ dev DEV ] [ label LABEL ]\n");
+	fprintf(stderr, "Usage: ip addrlabel { add | del } prefix PREFIX [ dev DEV ] [ label LABEL ]\n");
+	fprintf(stderr, "       ip addrlabel [ list | flush | help ]\n");
 	exit(-1);
 }