pg_combinebackup: Fix incorrect code documentation

pg_combinebackup: Fix incorrect code documentation

The code comment for parse_oid accidentally used the wrong parameter
when referring to the location of the last backup. Also, while there,
improve sentence wording by removing a superfluous word.

Backpatch to v17 where pg_combinebackup was addedd

Author: Amul Sul <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Discussion: https://postgr.es/m/CAAJ_b95ecWgzcS4K3Dx0E_Yp-SLwK5JBasFgioKMSjhQLw9xvg@mail.gmail.com
Backpatch-through: 17