HPCC-11803 Fix Typo in Sys Admin Guide

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
Merge pull request #6148 from JamesDeFabia/MiddlewareTypo

HPCC-11803 Fix Typo in Sys Admin Guide

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>