Update Cargo.lock due to mdbook extension changes

Author: ehuss

src/tools/rustbook/Cargo.lock

@@ -1,6 +1,6 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
-version = 3
+version = 4
 
 [[package]]
 name = "adler2"
@@ -704,6 +704,7 @@ dependencies = [
  "semver",
  "serde_json",
  "tempfile",
+ "walkdir",
 ]
 
 [[package]]