xrootd: Fix error message for read patch check

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7035/