Bug 10810 - Fix synopsis for -html option to overdue_notices.pl

Bug 10810 - Fix synopsis for -html option to overdue_notices.pl

The synopsis for the -html option used to be:
  -html <filename> Output html to file
but the argument to this option should actually be a directory,
not a filename. This patch fixes the synopsis.

To test:
- Run perldoc misc/cronjobs/overdue_notices.pl
- Check that the synopsis is as shown above
- Apply this patch
- Run perldoc misc/cronjobs/overdue_notices.pl
- Check that
  - the synopsis asks for a directory, not a filename
  - that the description of the -html option further down in the
    perldoc now mentions the filename that the HTML file will be
    created with.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Works as described. No koha-qa errors.

If feeded with a filename -html option fails with many messages like
print() on closed filehandle $html_fh at misc/cronjobs/overdue_notices.pl line 384.
Perhaps it could be improved, but that was the old behavior.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Improves documentation, no negative side effects found.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>