Fix some comments of do_pg_backup_start() and do_pg_backup_stop()
Both functions referred to an incorrect variable name, so make the whole
more consistent.
Oversight in
7d70809.
Author: Kyotaro Horiguchi, Bharath Rupireddy
Discussion: https://postgr.es/m/
20220927.172427.
467118514018439476.horikyota.ntt@gmail.com