docs: drop unnecessary out-of-frame target

I'm guessing an earlier version of the website used to have the page
contents in <frames>, but this isn't the case anymore so just drop the
unnecessary `target="_main"` :)

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
1 file changed