Bug 24055: (follow-up) Description of PayPalReturnURL system preference

Bug 24055: (follow-up) Description of PayPalReturnURL system preference

Have rephrased it as 'configured return URL' as this is what they call
it on the PayPal website.

To test:
- Go to Administration/ System preferences
- Search for PayPalReturnURL
- Verify description has changed to 'configured return URL' as one of
the options.

Sponsored.by: Catalyst IT

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>