add reference to MDN PR with the math & svg addition

Author: alexcjohnson

scripts/extract-elements.js

@@ -16,6 +16,7 @@ function extractElements($) {
         'html', 'head', 'body', 'style', 'h1–h6', 'input',
         // out of scope, different namespaces - but Mozilla added these to the
         // above reference page Jan 2021 so we need to exclude them now.
+        // see https://github.com/mdn/content/pull/410
         'svg', 'math',
         // obsolete, non-standard, or deprecated tags
         'image', 'dir', 'tt', 'applet', 'noembed', 'bgsound', 'menu', 'menuitem',