add decoration to admonition on latex/pdf
add decoration to admonition on latex/pdf

Signed-off-by: Yoshihiro Kaneko <fb9c50ad36156858cd51bb145369d6f181add714@gmail.com>

osrg/ryu-book,osrg/ryu-book,iwaseyusuke/ryu-book,iwaseyusuke/ryu-book