Update mdbook to 0.4.49

This is a routine update to pull in some fixes and updates.

Changelog: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0449

Author: ehuss

src/tools/rustbook/Cargo.lock

@@ -938,9 +938,9 @@ dependencies = [
 
 [[package]]
 name = "mdbook"
-version = "0.4.48"
+version = "0.4.49"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8b6fbb4ac2d9fd7aa987c3510309ea3c80004a968d063c42f0d34fea070817c1"
+checksum = "d1daacee059634081dee4250d2814763a365b92dfe14bfdef964bc27835209d4"
 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 = "0.4.48"
+version = "0.4.49"
 default-features = false
 features = ["search"]