org-agenda.el (org-entries-lessp): Fix inactive timestamp comparison

* org-agenda.el (org-entries-lessp): Fix inactive timestamp
comparison.

lisp/org-agenda.el

@@ -7022,7 +7022,7 @@ their type."
 			       (org-cmp-ts a b "deadline")))
 	 (deadline-down   (if deadline-up (- deadline-up) nil))
 	 (tsia-up         (and (org-em 'tsia-up 'tsia-down ss)
-			       (org-cmp-ts a b "iatimestamp_ia")))
+			       (org-cmp-ts a b "timestamp_ia")))
 	 (tsia-down       (if tsia-up (- tsia-up) nil))
 	 (ts-up           (and (org-em 'ts-up 'ts-down ss)
 			       (org-cmp-ts a b "timestamp")))