Updated tag content to match new GA instructions

[OlivierCodol]

The current instructions from the Google Analytics website are to use the following tag:

<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=GA_ID_HERE"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'GA_ID_HERE');
</script>

With GA_ID_HERE the website ID provided by google analytics.

This amends the current function that adds the tag to each webpage to include this tag instead of the previous tag, which was not working on the website instances I tested it with.