ox/ox-html: Fix defcustoms

ox/ox-html: Fix defcustoms

* lisp/ox.el (org-export-with-archived-trees, org-export-with-author,
  org-export-with-clocks, org-export-with-date,
  org-export-creator-string, org-export-with-drawers,
  org-export-with-email, org-export-with-emphasize,
  org-export-exclude-tags, org-export-with-fixed-width,
  org-export-with-footnotes, org-export-with-latex,
  org-export-headline-levels, org-export-default-language,
  org-export-preserve-breaks, org-export-with-entities,
  org-export-with-inlinetasks, org-export-with-planning,
  org-export-with-priority, org-export-with-section-numbers,
  org-export-select-tags, org-export-with-smart-quotes,
  org-export-with-special-strings,
  org-export-with-statistics-cookies,
  org-export-with-sub-superscripts, org-export-with-toc,
  org-export-with-tables, org-export-with-tags,
  org-export-with-tasks, org-export-time-stamp-file,
  org-export-with-timestamps, org-export-with-todo-keywords): Fix
  docstrings.
* lisp/ox-html.el (org-html-postamble-format): Slightly change default
  value so "Generated by" string doesn't get duplicated.
(org-html-creator-string): Fix docstring.