Update broken link (#35184)

Currently this link doesn't work (Service Unavailable). This PR updates the link

Author: urre

packages/gatsby-plugin-google-gtag/README.md

@@ -8,7 +8,7 @@ Global site tag (gtag.js) is meant to combine multiple Google tagging systems an
 
 For more general information on gtag you can read Google's official documentation on the subject: https://developers.google.com/gtagjs/.
 
-If you're migrating from analytics.js (gatsby-plugin-google-analytics) you can read about the subtle API differences in more depth at: https://developers.google.com/analytics/devguides/collection/gtagjs/migration.
+If you're migrating from analytics.js (gatsby-plugin-google-analytics) you can read about the subtle API differences in more depth at: https://developers.google.com/analytics/devguides/migration/ua/analyticsjs-to-gtagjs.
 
 NOTE: This plugin only works in production mode! To test your Global Site Tag is installed and firing events correctly run: `gatsby build && gatsby serve.`