Bug 36824: Fix conversion of __VERSION__ in system preferences to use main rather...

Bug 36824: Fix conversion of __VERSION__ in system preferences to use main rather than master

Test plan:
1. Administration > search system preferences for the words database columns
2. Click the links to schema.koha-community.org, note that they 404 with
   /master/ in the URL
3. Apply patch, reset_all
4. Search again, click the links, note that they load successfully with /main/

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>