ox-html: Fix "HTML_HEAD" and "HTML_HEAD_EXTRA" keywords

* lisp/ox-html.el (html): Fix "HTML_HEAD" and "HTML_HEAD_EXTRA"
  keywords.

lisp/ox-html.el

@@ -118,8 +118,8 @@
    (:html-mathjax "HTML_MATHJAX" nil "" space)
    (:html-postamble nil "html-postamble" org-html-postamble)
    (:html-preamble nil "html-preamble" org-html-preamble)
-   (:html-head "HTML_HEAD" nil nil org-html-head newline)
-   (:html-head-extra "HTML_HEAD_EXTRA" nil nil org-html-head-extra newline)
+   (:html-head "HTML_HEAD" nil org-html-head newline)
+   (:html-head-extra "HTML_HEAD_EXTRA" nil org-html-head-extra newline)
    (:html-head-include-default-style nil nil org-html-head-include-default-style)
    (:html-head-include-scripts nil nil org-html-head-include-scripts)
    (:html-table-tag nil nil org-html-table-tag)