ox-publish: Fix code typo

* lisp/ox-publish.el (org-publish-collect-index): Fix typo.

lisp/ox-publish.el

@@ -932,7 +932,7 @@ its CDR is a string."
 		     ((not parent) nil)
 		     ((let ((id (org-element-property :ID parent)))
 			(and id (cons 'id id))))
-		     ((let ((id (org-element-property :CUSTOM-ID parent)))
+		     ((let ((id (org-element-property :CUSTOM_ID parent)))
 			(and id (cons 'custom-id id))))
 		     (t (cons 'name
 			      (org-element-property :raw-value parent))))))))