Export back-ends: Update timestamp export
* contrib/lisp/org-e-ascii.el (org-e-ascii-timestamp): Update
timestamp export.
* contrib/lisp/org-e-groff.el (org-e-groff-timestamp): Update
timestamp export.
* contrib/lisp/org-e-html.el (org-e-html-timestamp): Update timestamp
export.
* contrib/lisp/org-e-latex.el (org-e-latex-timestamp): Update
timestamp export.
* contrib/lisp/org-e-odt.el (org-e-odt-timestamp): Update timestamp
export.
* contrib/lisp/org-e-texinfo.el (org-e-texinfo-timestamp): Update
timestamp export.