Update GitHub Actions actions/checkout@v2 to v3

The v2 implementation uses Node 12, which is end-of-life on April 30, 2022.
See https://nodejs.org/en/about/releases/. Update to v3, which is based on
Node 16 whose support lasts until April 30, 2024.

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: |