Improve HIG for Calc ttest Statistics .ui

Change-Id: I329ad988e9cfb12f41e8fd86af26c02fd0bef096
Reviewed-on: https://gerrit.libreoffice.org/10692
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>