Fix mkdocs workflow's command not found error

https://github.com/mhausenblas/mkdocs-deploy-gh-pages/pull/88

.github/workflows/mkdocs-deployment.yml

@@ -12,6 +12,6 @@ jobs:
       - name: Checkout main
         uses: actions/checkout@v2
       - name: Deploy docs
-        uses: mhausenblas/mkdocs-deploy-gh-pages@master
+        uses: mhausenblas/mkdocs-deploy-gh-pages@1.16
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}