Don't reference non-existent function (svn_fs_verify_root) in documentation.

svn_fs_verify_rev was renamed to svn_fs_verify_root in r1461648.

* subversion/include/svn_fs.h
  (svn_fs_verify): Fix docstring to reference svn_fs_verify_root.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483015 13f79535-47bb-0310-9956-ffa450edef68