change button to span due to Firefox compatibility issue

git-svn-id: https://plugins.svn.wordpress.org/the-events-calendar-shortcode/trunk@1017657 b8457f37-d9ea-0310-8a92-e5e31aec5664
change button to span due to FireFox compatibility issue

git-svn-id: https://plugins.svn.wordpress.org/the-events-calendar-shortcode/trunk@1017658 b8457f37-d9ea-0310-8a92-e5e31aec5664