fdo#83481 Correct syntax for external references with all formula syntax

Change-Id: I99ea9f026b95d36f6335414c46f332f5ba7cf235
Reviewed-on: https://gerrit.libreoffice.org/11286
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>