message: Rewrite example of Option::map_or
  Closes:
  Approved by: metajack
diff: @@ -498,10 +498,7 @@ fn run_transactions_naive(transactions: Vec<SignedTransaction>) -> HashValue {
  let mut iter = transactions.into_iter();
  let first_txn = iter.next();
  let response = block_on(executor.execute_block(
  - match first_txn {
  - None => vec![],
  - Some(txn) => vec![txn],
  - },
  + first_txn.map_or(vec![], |txn| vec![txn]),
  *GENESIS_BLOCK_ID,
  gen_block_id(1),
  ))
  