Fix usage info on svn-bench null-list and null-info

* subversion/svn-bench/svn-bench.c
  (svn_cl__cmd_table): Add missing "null-" in usage info
    for null-list and null-info

Obvious fix.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1637259 13f79535-47bb-0310-9956-ffa450edef68