Update mkdocs.yml

Try a new version of the checkout action.

.github/workflows/mkdocs.yml

@@ -8,7 +8,7 @@ jobs:
   linux:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up Python 3.8
       uses: actions/setup-python@v1
       with: