org-agenda.el: Fix org-agenda-with-point-at-orig-entry

* lisp/org-agenda.el (org-agenda-with-point-at-orig-entry): Add missing
  comma in the backquote template.

lisp/org-agenda.el

@@ -2006,7 +2006,7 @@ If STRING is non-nil, the text property will be fetched from position 0
 in that string.  If STRING is nil, it will be fetched from the beginning
 of the current line."
   (org-with-gensyms (marker)
-    `(let ((,marker (get-text-property (if string 0 (point-at-bol))
+    `(let ((,marker (get-text-property (if ,string 0 (point-at-bol))
 				       'org-hd-marker ,string)))
        (with-current-buffer (marker-buffer ,marker)
 	 (save-excursion