Bump mdbook from 0.4.37 to 0.4.40 (#21)

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.37 to 0.4.40.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.37...v0.4.40)

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

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