fix backup.config typo (#587)

Fixing https://github.com/github/ghes/issues/7329

Author: gamefiend

backup.config-example

@@ -26,7 +26,7 @@ GHE_NUM_SNAPSHOTS=10
 # performed before the next full backup is created.
 # For example, if `GHE_INCREMENTAL_BACKUP_MAX` is set to 14, backup-utils will 
 # run 1 full backup and then 13 incremental backups before performing another full backup on the next cycle.
-#GHE_INCREMENTAL_BACKUP_MAX=14
+#GHE_INCREMENTAL_MAX_BACKUPS=14
 
 # If GHE_SKIP_CHECKS is set to true (or if --skip-checks is used with ghe-backup) then ghe-host-check
 # disk space validation and software version checks on the backup-host will be disabled.