Fix workflow: use python3 -m mkdocs

Author: sun

.github/workflows/deploy-docs.yml

@@ -23,6 +23,6 @@ jobs:
         run: |
           git config --global user.name "github-actions[bot]"
           git config --global user.email "github-actions[bot]@users.noreply.github.com"
-          mkdocs gh-deploy --force
+          python3 -m mkdocs gh-deploy --force
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}