Update mdbook

Author: kateinoigakukun

.github/workflows/deploy.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v1
         with:
-          mdbook-version: '0.4.3'
+          mdbook-version: '0.5.1'
       - run: mdbook build
 
       - name: Deploy

book.toml

@@ -1,7 +1,6 @@
 [book]
 authors = ["Yuta Saito"]
 language = "en"
-multilingual = false
 src = "src"
 title = "Swift and WebAssembly"