Merge bitcoindevkit/bdk#622: fix typo

Merge bitcoindevkit/bdk#622: fix typo

a85ef62698eae748242f31db80fa3aabd3b2b64e fix typo (Buck Perley)

Pull request description:

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->

  ### Description

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->
  just a small typo fix

  ### Notes to the reviewers

  <!-- In this section you can include notes directed to the reviewers, like explaining why some parts
  of the PR were done in a specific way -->

  ### 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:
  rajarshimaitra:
    ACK a85ef62698eae748242f31db80fa3aabd3b2b64e
  afilini:
    ACK a85ef62698eae748242f31db80fa3aabd3b2b64e

Tree-SHA512: 089de23adae62492a0b39a27c9cb8cb8afc99e5634194118681b8a9a46ff0b073558f9cd515cd4db4c9c6e6f9c813bfa4b193d4e3f9558b34ad29cbd46cf028c