DOC: Fix link to "sphinx-doc" (#3488)

Author: yvonnefroehlich

doc/_templates/footer.html

@@ -3,7 +3,7 @@
 
 {%- block extrafooter %}
 <p>
-Built with <a href="https://sphinx-doc.org/">Sphinx</a>
+Built with <a href="https://www.sphinx-doc.org/">Sphinx</a>
 using a <a href="https://github.com/rtfd/sphinx_rtd_theme">theme</a>
 provided by <a href="https://readthedocs.org">Read the Docs</a>
 </p>