(bug 45978) Correct case of Special:Userrights' canonical name

Fix for I50796216 (17e17ba).

The error is PHP Notice:  Found alias defined for Userrights when
searching for special page aliases for UserRights. Case mismatch?

Bug: 45978
Change-Id: Ib82bd38d0229af7571f2ab3c0470fa0f0384b635