docs: use actions/checkout@v3

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout

README.md

@@ -20,7 +20,7 @@ jobs:
     name: runner / vale
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: errata-ai/vale-action@reviewdog
         env:
           # Required, set by GitHub actions automatically: