Bump mdbook from 0.4.41 to 0.4.43 (#59)

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.41 to 0.4.43.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.41...v0.4.43)

---
updated-dependencies:
- dependency-name: mdbook
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>