fdo#57950 Replace chained append in htmlatr.cxx

Change-Id: I121f437019d45f60017337dea091389ceb8334ce
Reviewed-on: https://gerrit.libreoffice.org/8661
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>