Fix stale links to x-govuk.github.io

Nanoc was flagging these old links, I've updated them to link to x-govuk.org instead

Author: DavidBiddle

guide/layouts/partials/header.slim

@@ -7,6 +7,6 @@ a.govuk-skip-link href="#main-content"
 header.govuk-header class=(@item[:headerClasses]) role="banner" data-module="govuk-header"
   .govuk-header__container.govuk-width-container
     .govuk-header__logo
-      a.govuk-header__link.govuk-header__link--homepage href="https://x-govuk.github.io"
+      a.govuk-header__link.govuk-header__link--homepage href="https://x-govuk.org"
 
         == render '/partials/x-govuk-logotype.*'

guide/lib/examples/footer_helpers.rb

@@ -36,7 +36,7 @@ def footer_with_custom_meta_html
         = render GovukComponent::FooterComponent.new do |footer|
           - footer.with_meta_html do
             | Built with love by
-            = govuk_footer_link_to("X-GOVUK", "https://x-govuk.github.io/")
+            = govuk_footer_link_to("X-GOVUK", "https://x-govuk.org/")
       FOOTER_META_HTML
     end
 
@@ -72,7 +72,7 @@ def footer_with_custom_licence_html
           - footer.with_meta_licence_html do
             |> Some custom HTML for the licence,
                perhaps with a
-            = govuk_footer_link_to("link", "https://x-govuk.github.io/")
+            = govuk_footer_link_to("link", "https://x-govuk.org/")
       FOOTER
     end