On the log-addressing branch:  Correct index structure doc.

* subversion/libsvn_fs_fs/structure-indexes
  (Proto index file format):  remove a misplaced negation

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1535693 13f79535-47bb-0310-9956-ffa450edef68