Bump mdbook from 0.4.1 to 0.4.2 in /blacksmith

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.1 to 0.4.2.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.1...v0.4.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

blacksmith/Cargo.lock

@@ -21,6 +21,6 @@ version = "2.33.2"
 default-features = false
 
 [dependencies.mdbook]
-version = "=0.4.1"
+version = "=0.4.2"
 default-features = false
 features = ["search"]