Merge bitcoindevkit/bdk#1668: docs: fix incorrect links to wallet examples

521b49d2da09e7c14c079970397f7d98f5cfe648 docs: fix incorrect links to wallet examples (Torkel Rogstad)

Pull request description:

  ### Description

  This PR updates the examples found in the wallet crate to use working links to GitHub.

  ### 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:
  ValuedMammal:
    ACK 521b49d2da09e7c14c079970397f7d98f5cfe648

Tree-SHA512: a4b214894c5a1453a2fe7b9732508e57c65793604440b7d8892f89cda3065cf850a44926b4c711738c13e13634d480dc75f67424edc4d3d09def328ef3b7042a