Avoid surprises by fixing GitHub Action version

.github/workflows/mkdocs.yml

@@ -6,14 +6,14 @@ on:
 
 jobs:
   build:
-    name: Deploy docs
+    name: Deploy MkDocs
     runs-on: ubuntu-latest
     steps:
       - name: Checkout master
         uses: actions/checkout@v2
 
-      - name: Deploy docs
-        uses: mhausenblas/mkdocs-deploy-gh-pages@master
+      - name: Deploy MkDocs
+        uses: mhausenblas/mkdocs-deploy-gh-pages@1.25
         # Or use mhausenblas/mkdocs-deploy-gh-pages@nomaterial to build without the mkdocs-material theme
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}