028caa9f8cd51afba0fdc81748fc8cf536c75cd0 fix(typos): existant -> existent (Jose Storopoli)
Pull request description:
### Description
These typos are blocking the Nix typo CI in #1257
### Notes to the reviewers
Blocking #1257
### Changelog notice
- fix: typos in documentation
-
### 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
#### New Features:
* [ ] I've added tests for the new feature
* [ ] I've added docs for the new feature
#### Bugfixes:
* [ ] This pull request breaks the existing API
* [ ] I've added tests to reproduce the issue which are now passing
* [ ] I'm linking the issue being fixed by this PR
ACKs for top commit:
notmandatory:
ACK 028caa9f8cd51afba0fdc81748fc8cf536c75cd0
Tree-SHA512: 0bd91efd0eec55fdc537824435552c968858a5b827179b3f9f3f37785db3fa92d3e6f0c73023de0c506224c81217c402d5afa8a2f768fecaf6a3c8378226d184
f6418ba911