ci: bump mdbook to latest

Author: nickelpro

.github/workflows/main.yaml

@@ -37,7 +37,7 @@ jobs:
       - name: Download mdBook
         run: |
           mkdir bin
-          curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.40/mdbook-v0.4.40-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+          curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.5.2/mdbook-v0.5.2-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
 
       - name: Build Docs
         run: bin/mdbook build