Bump mdbook from 0.4.11 to 0.4.12 in /blacksmith

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.11 to 0.4.12.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.11...v0.4.12)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

blacksmith/Cargo.lock

@@ -21,6 +21,6 @@ version = "2.33.3"
 default-features = false
 
 [dependencies.mdbook]
-version = "=0.4.11"
+version = "=0.4.12"
 default-features = false
 features = ["search"]