ox-html: Use correct "br" tag for subtitle

* lisp/ox-html.el (org-html-template): Use correct "br" tag depending
  on :html-doctype.

Reported-by: Olivier Berger <olivier.berger@telecom-sudparis.eu>
<http://permalink.gmane.org/gmane.emacs.orgmode/114546>

lisp/ox-html.el

@@ -2034,7 +2034,8 @@ holding export options."
 	      (format
 	       (if html5-fancy
 		   "<p class=\"subtitle\">%s</p>\n"
-		 "\n<br>\n<span class=\"subtitle\">%s</span>\n")
+		 (concat "\n" (org-html-close-tag "br" nil info) "\n"
+			 "<span class=\"subtitle\">%s</span>\n"))
 	       (org-export-data subtitle info))
 	    "")))))
    contents