Fix fdo#80436 Help pages for MODE functions

MODE.SNGL, MODE.MULT, Z.TEST, NEGBINOM.DIST

Change-Id: I0e6ad33f76a09c9d4af7b8146d07eb4f1df86313
Reviewed-on: https://gerrit.libreoffice.org/10411
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>