Merge pull request #3722 from alphagov/hreflang-mozilla-link

Link to MDN instead of w3schools on links page

src/styles/links/index.md

@@ -66,6 +66,6 @@ When offering links to content in other languages, make sure:
 
 - the link's text includes the name of the alternative language in both English and the source language
 - the link's purpose is always clear, even when taken out of context
-- the link element includes an [`hreflang` attribute](https://www.w3schools.com/tags/att_a_hreflang.asp) that identifies the language of the linked page.
+- the link element includes an [`hreflang` attribute](https://developer.mozilla.org/en-US/docs/Web/API/HTMLLinkElement/hreflang) that identifies the language of the linked page.
 
 For example, your link text could be 'use [Service name] in [language]'.