Remove reference to hgroup in ox-html

* ox-html.el (org-html-html5-elements): Drop reference to hgroup.

lisp/ox-html.el

@@ -170,10 +170,8 @@
     "progress" "section" "video")
   "New elements in html5.
 
-<hgroup> is not included because it's currently impossible to
-wrap special blocks around multiple headlines. For other blocks
-that should contain headlines, use the HTML_CONTAINER property on
-the headline itself.")
+For blocks that should contain headlines, use the HTML_CONTAINER
+property on the headline itself.")
 
 (defconst org-html-special-string-regexps
   '(("\\\\-" . "&#x00ad;")		; shy