Fix fdo#80446: Help entries for BETA.DIST BETA.INV

Change-Id: Ic8570dbe6cfe0bd5ed93ac43cd95470518abedc5
Reviewed-on: https://gerrit.libreoffice.org/9961
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>