Merge bitcoindevkit/bdk#1286: doc, example(bdk): fix derivation path in mnemonic_to_d...

d494f63d08f60ce01c22ff08f7e88a80a7be2522 doc, example(bdk): fix derivation path in mnemonic_to_descriptors (vmammal)

Pull request description:

  This will help avoid confusion with mainnet descriptors.

  ### 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:
  danielabrozzoni:
    ACK d494f63d08f60ce01c22ff08f7e88a80a7be2522

Tree-SHA512: f0450d171bc53085204280495f2954e20f4b64a73cfbcc9a0c3be131c0b04ad53e936c83fa83c89fbd1ed1c4edd680e8437550453f75056049d36f84cae1df43