org-latex.el: Allow "/" character in the #+LaTeX_CLASS option.

* org-latex.el (org-export-latex-set-initial-vars): Allow "/"
character in the #+LaTeX_CLASS option.

Thanks to Richard Lewis for a request about this.

lisp/org-latex.el

@@ -1346,7 +1346,7 @@ LEVEL indicates the default depth for export."
 	      (save-restriction
 		(widen)
 		(goto-char (point-min))
-		(and (re-search-forward "^#\\+LaTeX_CLASS:[ \t]*\\([-a-zA-Z]+\\)" nil t)
+		(and (re-search-forward "^#\\+LaTeX_CLASS:[ \t]*\\([-/a-zA-Z]+\\)" nil t)
 		     (match-string 1))))
 	    (plist-get org-export-latex-options-plist :latex-class)
 	    org-export-latex-default-class)