Merge bitcoindevkit/bdk#1458: fix: typo on `SignedAmount` instead of `Amount`

Merge bitcoindevkit/bdk#1458: fix: typo on `SignedAmount` instead of `Amount`

20341a3ca1ab85ad77e022b5028136a635c3f42c fix: typo on `SignedAmount` instead of `Amount` (Leonardo Lima)

Pull request description:

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  ### Description

  It fixes the typo on the `expect()` message introduced on #1426, noticed at https://github.com/bitcoindevkit/bdk/pull/1426#discussion_r1626761825

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

  ### 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:
  storopoli:
    ACK 20341a3ca1ab85ad77e022b5028136a635c3f42c
  notmandatory:
    ACK 20341a3ca1ab85ad77e022b5028136a635c3f42c

Tree-SHA512: 62deb7308b2a6891eeb4598ebdf3c5ee1541fc52fd454bca2816b819bd7f6ab25c18e10c4166c80c4e553bcb3ce2207323376d260484a956837ac857854cbd6b