fdo#86871: better button label in Index/Contents

Change the ellipsis button label
Insert -> Table of Contents -> Index and Table of contents tab -> Create from -> Styles -> ...
This last ellipsis means actually "Assign styles...".

Change-Id: I22d761891d212240aedfe26ce3885b1ded2dcc1a
Reviewed-on: https://gerrit.libreoffice.org/13207
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>