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 <sulamul@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CAAJ_b95ecWgzcS4K3Dx0E_Yp-SLwK5JBasFgioKMSjhQLw9xvg@mail.gmail.com
Backpatch-through: 17

src/bin/pg_combinebackup/pg_combinebackup.c

@@ -804,7 +804,7 @@ parse_oid(char *s, Oid *result)
  * Copy files from the input directory to the output directory, reconstructing
  * full files from incremental files as required.
  *
- * If processing is a user-defined tablespace, the tsoid should be the OID
+ * If processing a user-defined tablespace, the tsoid should be the OID
  * of that tablespace and input_directory and output_directory should be the
  * toplevel input and output directories for that tablespace. Otherwise,
  * tsoid should be InvalidOid and input_directory and output_directory should
@@ -816,7 +816,7 @@ parse_oid(char *s, Oid *result)
  *
  * n_prior_backups is the number of prior backups that we have available.
  * This doesn't count the very last backup, which is referenced by
- * output_directory, just the older ones. prior_backup_dirs is an array of
+ * input_directory, just the older ones. prior_backup_dirs is an array of
  * the locations of those previous backups.
  */
 static void