Use action/checkout@v3 in the book

Signed-off-by: Yuki Okushi <[email protected]>

Author: JohnTitor

book/src/continuous_integration/github_actions.md

@@ -15,7 +15,7 @@ jobs:
   clippy_check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v3
       - name: Run Clippy
         run: cargo clippy --all-targets --all-features
 ```