Misspelled property in SwAuthenticationSettingsDialog (seperate => separate)

Change-Id: I0cf29711459e5ec185b43cf28716729e659b00b4
Reviewed-on: https://gerrit.libreoffice.org/9357
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>