[docs] how-to-submit-a-transaction.md suggestion / change request

[b1bart]

File: [docs/how-to-guides/01_how-to-submit-a-transaction.md](https://github.com/EOSIO/eosjs/blob/develop/docs/how-to-guides/01_how-to-submit-a-transaction.md)

This example and the README use a blocksBefore of 3. It seems like useLastIrreversible is a safer default for all how-to guides with the blocksBefore being something discussed as an additional option.

This has the added benefit of generating the refBlockNum and refBlockPrefix based on the results of get_info rather than issuing an additional get_block