Bug 12231 - Update AllowMultipleIssuesOnABiblio language

Bug 12231 - Update AllowMultipleIssuesOnABiblio language

This preference had misleading language, this patch changes it a bit
to make it clearer.

To test:
* Apply patch
* Visit System Preferences
* Go to Circulation Tab
* Read description of AllowMultipleIssuesOnABiblio preference
* Confirm that it is clearer

http://bugs.koha-community.org/show_bug.cgi?id=12231
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>

String patch, no errors

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>