$(this)
is actually pointing to <h3 class="showpageblock">
and not to <span class="entypo chevron-down"></span>
which is inside the "h3" tag.
change this
$(this).toggleClass("entypo chevron-right");
to
$(this).find('span').toggleClass("entypo chevron-right");
LIVE DEMO:
http://jsfiddle.net/dreamweiver/5umDs/2/
Happy Coding :)