Small style update to improve Markdown rendering

Signed-off-by: Skia <skia@libskia.so>

core/templates/core/pagerev_edit.jinja

@@ -26,7 +26,7 @@ function make_preview() {
     <p><input type="submit" value="{% trans %}Save{% endtrans %}" /></p>
 </form>
 <a href="{{ url('core:page_delete', page_id=page.id)}}">{% trans %}Delete{% endtrans %}</a>
-<div id="preview">
+<div id="preview" class="page_content">
 </div>
 {% endblock %}