Merge bitcoindevkit/bdk#1086: docs: fix spelling errors

Merge bitcoindevkit/bdk#1086: docs: fix spelling errors

94f8fa530b34ae1c099db3df070000121e8283cf docs: fix spelling errors (Steve Myers)

Pull request description:

  ### Description

  I did a global spell check and found and fixed a few spelling errors.

  ### Notes to the reviewers

  This is low priority but want to make sure it's done before we do a beta release.

  ### Changelog notice

  None.

  ### 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:
  realeinherjar:
    ACK 94f8fa5
  evanlinjin:
    ACK 94f8fa530b34ae1c099db3df070000121e8283cf

Tree-SHA512: eed0be31264c45d8ef4d6b77e30927591014a76d71696f324440989e77b488d6bf018002d6df2354575b25e5a50d49bca6592976a310e6de7009e44d6b35518d