fix(examples): make regtest default on wallet_example_rpc

[luisschwab]

Description

This PR makes Regtest the default network on wallet_example_rpc and updates the README accordingly.

Notes to the reviewers

It had Testnet as the default network, 8332 as the default port and 481824 as the starting height; now everything is Regtest.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[oleonardolima]

cACK 710b9cc

I personally prefer to use regtest on the examples too, let's see what others think about it.

Also, you'd need a rebase to make sure that CI is running properly.

[notmandatory]

ACK 8ed817e

I agree, especially for the RPC client default network should be Regtest so user doesn't need to wait for testnet or mainnet IBD.

[ValuedMammal]

ACK 8ed817e