Fix warning in TAP test of pg_verifybackup

Oversight in a3fcbcd.

Reported-by: Thomas Munro
Discussion: https://postgr.es/m/CA+hUKGKnajZEwe91OTjro9kQLCMGGFHh2vvFn8tgHgbyn4bF9w@mail.gmail.com
Backpatch-through: 13

src/bin/pg_verifybackup/t/007_wal.pl

@@ -64,7 +64,7 @@ command_fails_like(
 # a timeline > 1.  Rather than plugging in a new standby, do a
 # self-promotion of this node.
 $primary->stop;
-$primary->append_conf('standby.signal');
+$primary->append_conf('standby.signal', '');
 $primary->start;
 $primary->promote;
 $primary->safe_psql('postgres', 'SELECT pg_switch_wal()');