Update checkout to new version v3

.github/workflows/docs-update.yml

@@ -12,8 +12,8 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
+      - uses: actions/checkout@v3
         with:
           repository: gweicz/utxo22-docs
           ref: master