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