Merge bitcoindevkit/bdk#599: Fix typo in docs for TxBuilder allow_shrinking method

4628a10191f451a7b0ade674e9f8568bf695e49c Fix typo in docs for TxBuilder allow_shrinking method (thunderbiscuit)

Pull request description:

  ### Description
  This PR fixes a small typo in the documentation for the `allow_shrinking()` method on the `TxBuilder`.

  ### Notes to the reviewers
  The sentence
  ```txt
  Explicitly tells the wallet that it is allowed to reduce the fee of the output matching this `script_pubkey` in order to bump the transaction fee. Without specifying this the wallet will attempt to find a change output to shrink instead.
  ```
  was changed for
  ```txt
  Explicitly tells the wallet that it is allowed to reduce the amount of the output matching this `script_pubkey` in order to bump the transaction fee. Without specifying this the wallet will attempt to find a change output to shrink instead.
  ```

  To reflect the fact that it's the _amount_ of the output that is being reduced in order to leave more for the fees.

  ### 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 4628a10191f451a7b0ade674e9f8568bf695e49c

Tree-SHA512: ca135764d78144295fdbdb4673ca6dec153c6f83ee7fb5d7d3051e0d46b04d23b2f3c3fd66e44491da842c778e72b52f8996d00e4c20986d7ccb11587f0d7d27