srm: Fix scheduling period for expiration task

I believe the intention was to schedule the task every 60 seconds, not every
60 milliseconds.

Target: trunk
Request: 2.8
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6816/
(cherry picked from commit 8d4b4bd7adf6f85989d21a4b3a7c71f1ea987f3c)