Add missing :group keyword in defcustom

* org-html.el (org-export-html-protect-char-alist): Add missing :group
keyword in defcustom.

lisp/org-html.el

@@ -565,6 +565,7 @@ a file."
     ("<" . "&lt;")
     (">" . "&gt;"))
   "Alist of characters to be converted by `org-html-protect'."
+  :group 'org-export-html
   :type '(repeat (cons (string :tag "Character")
 		       (string :tag "HTML equivalent"))))