tools: update checkout action on CI

https://github.com/actions/checkout\#whats-new

Author: UltiRequiem

.github/workflows/contrib.yml

@@ -8,7 +8,7 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
         fetch-depth: 0
     - name: Install mdbook

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
   rustfmt:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - run: rustup update stable && rustup default stable
     - run: rustup component add rustfmt
     - run: cargo fmt --all --check