Use correct variable when calculating `org-timer-start-time'

* org-timer.el (org-timer-start): Use correct variable when
calculating `org-timer-start-time'.

lisp/org-timer.el

@@ -102,7 +102,7 @@ the region 0:00:00."
 	  (setq delta (org-timer-hms-to-secs (org-timer-fix-incomplete s)))))
 	(setq org-timer-start-time
 	      (seconds-to-time
-	       (- (org-float-time) (org-timer-hms-to-secs s)))))
+	       (- (org-float-time) delta))))
       (org-timer-set-mode-line 'on)
       (message "Timer start time set to %s, current value is %s"
 	       (format-time-string "%T" org-timer-start-time)