html_formatter: fix link formatting
html_formatter: fix link formatting

We want to link to the id of the containing div, not to the
id computed for the title contained in this div.

The intention of the original patch is preserved, namely linking
to a fragment instead of the full address of the div.

thiblahute/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc