message: fix: Difficulty verify typo
diff: @@ -15,7 +15,7 @@ pub trait PowEngine: Send + Sync {
  fn verify_header(&self, header: &Header) -> bool {
  let proof_hash: H256 = blake2b(&header.seal.proof).into();
  - if boundary_to_difficulty(&proof_hash) >= header.difficulty {
  + if boundary_to_difficulty(&proof_hash) < header.difficulty {
  return false;
  }
  