fdo#71350 fix incorrect calculation of Excel function CONFIDENCE.T

Change-Id: I94202bebcc7deded51636f4bf7f203254a31931f
Reviewed-on: https://gerrit.libreoffice.org/6751
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>