Update mdbook to 0.4.52

Changelog: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0452

This primarily picks up a few fixes.

src/tools/rustbook/Cargo.lock

@@ -885,9 +885,9 @@ dependencies = [
 
 [[package]]
 name = "mdbook"
-version = "0.4.51"
+version = "0.4.52"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a87e65420ab45ca9c1b8cdf698f95b710cc826d373fa550f0f7fad82beac9328"
+checksum = "93c284d2855916af7c5919cf9ad897cfc77d3c2db6f55429c7cfb769182030ec"
 dependencies = [
  "ammonia",
  "anyhow",

src/tools/rustbook/Cargo.toml

@@ -15,6 +15,6 @@ mdbook-i18n-helpers = "0.3.3"
 mdbook-spec = { path = "../../doc/reference/mdbook-spec" }
 
 [dependencies.mdbook]
-version = "0.4.51"
+version = "0.4.52"
 default-features = false
 features = ["search"]