Bug 25945: Description of AuthoritySeparator is misleading

Bug 25945: Description of AuthoritySeparator is misleading

This patch changes the description of the AuthoritySeparator system preference

To test:
1. Apply patch
2. Go to Administration > Global system preferences > Cataloging and locate the
AuthoritySeparator preference
3. Read the description and make sure it makes sense and that there are no typos
or grammatical errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>