On the 'move-tracking-2' branch: Correct a doc string.

* subversion/svnmover/svnmover.c
  (svn_branch_branch): Fix doc string.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1627477 13f79535-47bb-0310-9956-ffa450edef68