org-bibtex.el: Fix broken URL

* lisp/org-bibtex.el: In the documentation section of the file, fix
  the broken URL to Andrew Roberts' document on BibTeX entries.

TINYCHANGE

lisp/org-bibtex.el

@@ -88,7 +88,7 @@
 ;;
 ;; - All Bibtex information is taken from the document compiled by
 ;;   Andrew Roberts from the Bibtex manual, available at
-;;   http://www.andy-roberts.net/misc/latex/sessions/bibtex/bibentries.pdf
+;;   http://www.andy-roberts.net/res/writing/latex/bibentries.pdf
 ;;
 ;;; History:
 ;;