lisp/ox-html.el: Finish the headline numbering part by a full stop

* lisp/ox-html.el (org-html-headline): Finish the numbering part
with a full stop, to be consistent with the TOC numbers.

TINYCHANGE

Link: https://orgmode.org/list/2F46C11A-B2CD-445C-A811-CB05A043FE46@dong.mk

lisp/ox-html.el

@@ -2646,7 +2646,7 @@ holding contextual information."
                                 (format
                                  "<span class=\"section-number-%d\">%s</span> "
                                  level
-                                 (mapconcat #'number-to-string numbers ".")))
+                                 (concat (mapconcat #'number-to-string numbers ".") ".")))
                            formatted-text)
                           level)
                   ;; When there is no section, pretend there is an