[web-animations-1] Point MathJax script to latest 2.x version

Author: birtles

web-animations-1/Overview.bs

@@ -233,7 +233,7 @@ spec:infra; type:dfn; for:list; text:extend
 <script type="text/x-mathjax-config">
   MathJax.Hub.Config({showMathMenu: false});
 </script>
-<script src="https://www.w3.org/scripts/MathJax/2.6.1/MathJax.js?config=MML_SVG"></script>
+<script src="https://www.w3.org/scripts/MathJax/2/MathJax.js?config=MML_SVG"></script>
 
 Introduction {#introduction}
 =======================================