ox-html: Fix center block class

* ox-html.el (org-html-center-block): Correct class.

Reported-by: j_l_domenech@yahoo.com
<http://permalink.gmane.org/gmane.emacs.bugs/105805>

lisp/ox-html.el

@@ -2305,7 +2305,7 @@ contextual information."
   "Transcode a CENTER-BLOCK element from Org to HTML.
 CONTENTS holds the contents of the block.  INFO is a plist
 holding contextual information."
-  (format "<div class=\"center\">\n%s</div>" contents))
+  (format "<div class=\"org-center\">\n%s</div>" contents))
 
 ;;;; Clock