Update mdbook to 0.4.51

This fixes a silly regression introduced in 0.4.50 that broke the search
hotkey (rust-lang#141457).

Changelog: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0451

Author: ehuss

src/tools/rustbook/Cargo.lock

@@ -885,9 +885,9 @@ dependencies = [
 
 [[package]]
 name = "mdbook"
-version = "0.4.50"
+version = "0.4.51"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f72bc08f096e1fb15cfc382babe218317c2897d2040f967c4db40d156ca28e21"
+checksum = "a87e65420ab45ca9c1b8cdf698f95b710cc826d373fa550f0f7fad82beac9328"
 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 = "0.4.50"
+version = "0.4.51"
 default-features = false
 features = ["search"]