fix detection of Salesforce html

Change-Id: Ie777c253e221527cba8affaf766e2eddfc78c103
Reviewed-on: https://gerrit.libreoffice.org/10012
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>