Bug 31931: Terminology for HoldsSplitQueue - use staff interface

Bug 31931: Terminology for HoldsSplitQueue - use staff interface

Rename "..staff client.." to "..staff interface.." in the description for the
HoldsSplit Queue system preference, as per the terminology guidelines.

Test plan.
1. View the description for the HoldsSplitQueue system preference.
2. Note that the description says "..staff client..".
3. Apply the patch.
4. Note that the description now says "..staff interface.." as per the
   terminology guidelines (https://wiki.koha-community.org/wiki/Terminology#S).
5. Sign off :D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>