update plotly ref in docs template

Author: kMutagene

docs/_template.html

@@ -26,7 +26,7 @@
 
     <link rel="shortcut icon" type="image/x-icon" href="{{root}}/img/favicon.ico">
     <script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
-    <script src="https://cdn.plot.ly/plotly-2.18.1.min.js"></script>
+    <script src="https://cdn.plot.ly/plotly-2.21.0.min.js"></script>
     <script>MathJax = { tex: { inlineMath: [['$', '$'], ['\\(', '\\)']] } };</script>
     <script src="https://cdn.jsdelivr.net/npm/[email protected]/es5/tex-svg.js"></script>