replace goerli with sepolia in the readme.md

[cocoyeal]

Since goerli testnet has been deprecated for one month, the "goerli" command in readme.md no longer works. so i replaced it with "sepolia".

[fjl]

We need to make a bigger change there, because simply running geth like this won't work. Sepolia also requires a beacon client to run.

[cocoyeal]

@fjl Hi sir, after doing some investigation, i found that the holesky will replace goerli as a staking, infrastructure and protocol-developer testnet. should we use holesky here instead of sepolia?

[fjl]

The main problem is that running Geth without a CL or at least a beacon chain endpoint is not possible anymore. I personally don't care if we put Sepolia or Holesky, it'd be more important to come up with new text that explains how to connect to the testnet with a CL as well.