Bug 11458: Improve confusing description of syspref 'gist'

Bug 11458: Improve confusing description of syspref 'gist'

The description of "gist" was:

"Default tax rates are ... (enter in numeric form, 0.12 for 12%.
First is the default. If you want more than 1 value, please
separate with |) "

The doubled use of "default" is confusing here.

With the patch it reads:

Tax rates are ... Enter in numeric form, 0.12 for 12%.
The first item in the list will be selected by default.
For more than one value, separate with | (pipe)

To test:
- Verify that the gist system preference description is
  correct.

The use of "default" is confusing here.

Signed-off-by: Aleisha <aleishaamohia@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>