ox-html: Fix org-html-format-drawer-function's docstring

* lisp/ox-html.el (org-html-format-drawer-function): Drop leftover
text from when an example used to be included.

adcebf38f (Fix errors reported by cus-test.el, 2013-11-14) dropped the
example but left the leading part.

Reported-by: Jean-Baptiste Mazon <jmazon@gmail.com>
Ref: https://orgmode.org/list/0e5569e6-15a7-d4c4-0558-8b0ef96a515c@gmail.com

lisp/ox-html.el

@@ -653,9 +653,6 @@ The function must accept two parameters:
 
 The function should return the string to be exported.
 
-For example, the variable could be set to the following function
-in order to mimic default behavior:
-
 The default value simply returns the value of CONTENTS."
   :group 'org-export-html
   :version "24.4"