Merge bitcoindevkit/bdk#1872: fix(examples): make regtest default on wallet_example_rpc

Merge bitcoindevkit/bdk#1872: fix(examples): make regtest default on wallet_example_rpc

8ed817e716f1b4d473f1c6ec29e11e1f7c0e9356 fix: make regtest default on wallet_example_rpc (Luis Schwab)

Pull request description:

  ### 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:

  * [X] I've signed all my commits
  * [X] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [X] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  notmandatory:
    ACK 8ed817e716f1b4d473f1c6ec29e11e1f7c0e9356
  ValuedMammal:
    ACK 8ed817e716f1b4d473f1c6ec29e11e1f7c0e9356

Tree-SHA512: 9fc2d3053a3ad6012f3e2494ae9cb3cb2217173f02ab5b1defb7ad9bf141df5ae64459cb4d3291f8dfec8f8d3f667e48d21d476c001fb717abfe6cfa26869202