pgsql: Fix warning in TAP test of pg_verifybackup

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

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ae254356f94af220841f3c59dcd4d820449ca0fc

Modified Files
--------------
src/bin/pg_verifybackup/t/007_wal.pl | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)