doc update for chainId

`chainId` defaults to `eth_chainId` rpc call but its not correct in doc [here](https://web3js.readthedocs.io/en/v1.5.2/web3-eth-accounts.html#id4) :

`chainId - String: (optional) The chain id to use when signing this transaction. Default will use web3.eth.net.getId().` which is `net_version`

so need doc correction