Add missing es5 directory to script src

Author: dpvc

README.md

@@ -51,7 +51,7 @@ need to install anything.  Simply use a `script` tag that loads
 MathJax from the CDN.  E.g.,
 
     <script id="MathJax-script" async
-      src="https://cdn.jsdelivr.net/npm/mathjax@3/tex-mml-chtml.js">
+      src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">
     </script>
 
 See the [MathJax