Update example workflow in readme

Author: jonasbb

README.md

@@ -24,7 +24,7 @@ jobs:
     name: cargo fmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       # Ensure rustfmt is installed and setup problem matcher
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with: