ci(markdownlint): upgrade actions/checkout to v3 (chrislgarry#877)

Author: oscard0m

.github/workflows/markdownlint.yml

@@ -5,6 +5,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Check out code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
     - name: Run mdl
       uses: actionshub/[email protected]