Add “summary” to ‘org-html-html5-elements’

* lisp/ox-html.el (org-html-html5-elements): Add “summary”

As suggested by Kaushal Modi in
<https://lists.gnu.org/archive/cgi-bin/namazu.cgi?query=%2Bmessage-id%3A%3CCAFyQvY2YoLDpM9PWLV86FwvPOVkuxg9O8KdryG3z%3D5QRm7Aujg%40mail.gmail.com%3E&submit=Search&idxname=emacs-orgmode>

lisp/ox-html.el

@@ -215,7 +215,7 @@
 (defconst org-html-html5-elements
   '("article" "aside" "audio" "canvas" "details" "figcaption"
     "figure" "footer" "header" "menu" "meter" "nav" "output"
-    "progress" "section" "video")
+    "progress" "section" "summary" "video")
   "New elements in html5.
 
 For blocks that should contain headlines, use the HTML_CONTAINER