Update checkout action to v3 (#1392)

Author: Philippus

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
   build-and-validate-docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: '2.6.9'