fix plotly CDN link

pywebio/platform/tpl/index.html

@@ -77,7 +77,7 @@
 
     require.config({
         paths: {
-            'plotly': "https://cdn.plot.ly/plotly-latest.min",
+            'plotly': "https://cdn.plot.ly/plotly-2.12.1.min",
         },
     });