Bug 35596: Error in writeoff_debts documentation

Bug 35596: Error in writeoff_debts documentation

This patch corrects an error in the documentation of the
writeoff_debts.pl script. The --type parameter accepts debit type codes,
not credit type codes.

To test:
-  Run ./misc/cronjobs/writeoff_debts.pl
-  Make sure the sentence for --type makes sense, grammar and spelling
   are correct

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>