org-clock: Correct file name in header
* lisp/org-clock.el (org-clock-save): Use the variable
org-clock-persist-file as the file name in the header rather than
hardcoding it to "org-persist.el".
From the introduction of org-clock-save in 6ca205398 (Add clock
persistence., 2008-10-23), the hardcoded file name did not match the
value of org-clock-persist-file.