Merge pull request #262 from dtolnay-contrib/checkoutv3

Update GitHub Actions actions/checkout@v2 to v3

Author: dtolnay

.github/workflows/deploy.yml

@@ -9,7 +9,7 @@ jobs:
     name: GitHub Pages
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Get mdBook
         run: |