Merge bitcoindevkit/bdk#478: Fix typos in comments

Merge bitcoindevkit/bdk#478: Fix typos in comments

7f8103dd762572f8b0f01514d0c32723d27c61e9 Fix typos in comments (thunderbiscuit)

Pull request description:

  ### Description

  This PR fixes a bunch of small typos in comments. I'm getting acquainted with the codebase and found a few typos just by chance, and ended up going through it with an IDE searching for typos in all files.

  ### Notes to the reviewers

  To be clear, this PR _only addresses typos that are within comments_.

  ### Checklists

  * [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 7f8103dd762572f8b0f01514d0c32723d27c61e9

Tree-SHA512: eb3f8f21cbd05de06292affd9ef69c21b52022dfdf25c562c8f4d9c9c011f18175dff0c650cb7efcfb2b665f2af80d9a153be3d12327c47796b0d00bfd5d9803