add sphinx_rtd_theme to sphinx workflow

Author: alperaltuntas

.github/workflows/sphinx.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Install dependencies
       run: conda install -y sphinx_rtd_theme
     - name: Build HTML
-    - uses: ammaraskar/sphinx-action@master
+      uses: ammaraskar/sphinx-action@master
       with:
         pre-build-command: "conda install -y sphinx_rtd_theme"
     - name: Upload artifacts