Fix link to 'Gutenberg Design Principles' on the Gutenberg page

[garrett-eclipse]

Hello,

Zeb identified this in Slack but I didn't see a ticket come through so wanted to create before it left my brain.

On this page - ​https://wordpress.org/gutenberg/

The link to Gutenberg Design Principles links to a non-WP url that redirects to the official page. We should update the link to use the official URL;
​https://wordpress.org/gutenberg/handbook/reference/design-principles/

The content there is hardcoded as part of the theme, in meta, the content is here:
https://meta.trac.wordpress.org/browser/sites/trunk/wordpress.org/public_html/wp-content/themes/pub/gutenberg/gutenfront-content.php

Cheers

[garrett-eclipse]

Replace URL with official one

[garrett-eclipse]

I've uploaded 3960.patch to address this minor URL fix.
I haven't done much meta contrib so please correct me if I'm _doing_it_wrong

Thanks

[dd32]

In 7906:

Gutenberg WordPress.org Theme: Update the documentation to point to the official location.

Props garrett-eclipse.
Fixes #3960.