Improve document table of content style (shader-slang#1812)

Author: csyonghe

docs/_layouts/user-guide.html

@@ -33,7 +33,7 @@
 
     .toc_list {
       list-style-type: none;
-      padding-left:16px;
+      padding-left:20px;
     }
   </style>
   {% seo %}