doc/api/libbabeltrace2/style.css: make font weight of `.intertd` normal

doc/api/libbabeltrace2/style.css: make font weight of `.intertd` normal

For some reason, `.intertd` paragraphs, which Doxygen creates when a
given table cell (used for parameter descriptions, amongst other things)
contains more than one paragraphs, are bold.

Here's an example to generate such paragraphs:

    @param some_param
        @parblock
        Normal paragraph.

        Bold paragraph.

        Also bold.
        @endparblock

I don't know the bold font weight's rationale here, but it does not look
pretty, so force the weight to be normal in `style.css`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ida6970a75f6cc1b4ac4f1fc873e86a3b453d09ac
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3225