org-odt.el: Don't corrupt styles.xml while applying outline numbering
* contrib/lisp/org-odt.el
(org-odt-configure-outline-numbering): Tighten the regular
expression that matches "<text:outline-level-style ...>"
element.
Fixes bug reported here
http://lists.gnu.org/archive/html/emacs-orgmode/2011-10/msg00594.html