On the invoke-diff-cmd branch: Fix typo in doc string.

* subversion/include/svn_client.h
  (svn_client_diff_peg6): Fix typo.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1484625 13f79535-47bb-0310-9956-ffa450edef68