Fixed typo in logback configuration

Target: trunk
Request: 2.7
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6234/
(cherry picked from commit 000c505f2d8c8bcf91e65d668ddcdb2a52638a85)