Fix a copy-pasted docstring.

* subversion/include/svn_repos.h
  (svn_repos_notify_t.warning): Remove a sentence that doesnt make sense for
    an enum- (rather than svn_error_t *) typed member.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1478617 13f79535-47bb-0310-9956-ffa450edef68