Bug 24055: Rephrase description of PayPalReturnURL slightly

Bug 24055: Rephrase description of PayPalReturnURL slightly

changes 'OPAC's alias' to 'specified URL' as suggested by
Tomas on comment#2.

To test:
- Go to Administration / System preferences
- Search for PayPalReturnURL
- Verify description has changed to "specified URL" as
  one of the options

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>