Fix badname option to be singular just like useradd.

Badnames still accepted, note that previously usage already stated
singular form, whilst manpage and real one was plural only.

Fixes: 45d6746219 ("src: correct "badname" option")

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>

src/usermod.c

@@ -980,6 +980,7 @@ static void process_flags (int argc, char **argv)
 		int c;
 		static struct option long_options[] = {
 			{"append",       no_argument,       NULL, 'a'},
+			{"badname",      no_argument,       NULL, 'b'},
 			{"badnames",     no_argument,       NULL, 'b'},
 			{"comment",      required_argument, NULL, 'c'},
 			{"home",         required_argument, NULL, 'd'},